Limits to appearance of a phantom steed

In a recent game session, a player wanted to become invisible and then use a phantom steed to quickly and stealthfully pass through the enemy camps surrounding a besieged city. This began a discussion of whether the phantom steed summoned could be transparent, or nearly so. The spell says:

A Large quasi-real, horselike creature appears on the ground in an unoccupied space of your choice within range. You decide the creature’s appearance…The creature uses the statistics for a riding horse

So what are the limits to the appearance of the steed? While "quasi-real", can it be made to appear so real that an observer would believe it to be a real horse? Can it be made to appear so unreal as to be seen through and receive a bonus on Stealth?

For the limit on how transparent it can be I thought the important restriction is that it has the stat block of a riding horse. Any permanent bonus to Stealth would be included within this block, and a riding horse does not have one. Thus, for me the mechanical limit to transparency would be that it could get a DM-assigned circumstantial bonus (advantage to Stealth checks depending on situation) but not a permanent bonus irrespective of circumstances (modification of stat block).

For the limit on how real it can appear, I looked to the spell Major Image, as an Illusion spell of equal level, whose illusions have the following property

A creature that uses its action to examine the image can determine that it is an illusion with a successful Intelligence (Investigation) check against your spell save DC.

Thus if the steed was summoned to appear real, it would fool any observer who was not explicitly trying to determine its nature, and even some of those depending on the roll. This approach of comparing the spell to same-school spells of equal level comes to me from 1e, with which I have more experience, and in which spells are much less clearly defined in terms of their limits. 5e, on the other hand, has the general principle that "spells do what they say they do". Is such a comparison to another spell then valid?

1. What are the limits to the appearance of a phantom steed? How real or unreal can it appear?

2. When deciding (1), is it useful to compare the spell to Major Image?

Do I need to feed a mount summoned by Find Steed?

Find Steed reads:

You summon a spirit that assumes the form of an unusually intelligent, strong, and loyal steed, creating a long-lasting bond with it. Appearing in an unoccupied space within range, the steed takes on a form that you choose, such as a Warhorse, a pony, a camel, an elk, or a Mastiff. (Your DM might allow Other Animals to be summoned as steeds.) The steed has the Statistics of the chosen form, though it is a celestial, fey, or fiend (your choice) instead of its normal type. […]

That mount is a spirit, not a common animal of that type. Since the spell doesn’t go into details, I assume I don’t have to feed it as I’d have to feed a warhorse, for instance. However, do spirits need to eat in D&D?

The reason being: although celestials/fey/fiends most likely eat unless otherwise specified (ex: angels), they can be killed by bringing them to 0 HP.
On the other hand, a mount from Find Steed “disappears leaving no physical form” and can be brought back fully restored — basically, if it drops to 0 HP its body simply fades and it won’t die.
That fits the spirit part in the spell’s description and strongly indicates that it’s not just fluff, as it doesn’t work exactly like the usual celestials / fey / fiends nor the base animal.

The summoned spirit might come in the form of a celestial horse, but that doesn’t imply it actually eats like one, just like it doesn’t die like an ordinary horse or celestial.

The question: is there any factual or explicit rule to conclusively support a “yes, it eats/no, it doesn’t” answer?

Gigantic Steed class ability gives all the benefits of a Huge mount?

As per title, I’m wondering whether the Gigantic Steed ability of the Mammoth Rider grants your mount all the benefits of a size increase, namely +8 Str. In fact, the text of the ability is:

A mammoth rider’s steed increases to Huge size. […] It also gains a +2 size bonus to its Strength and Constitution.

And then the bonuses to Str and Con increase over time with the Mammoth Rider class. So, for example, a standard Mastodon companion has a base str of 24, so by becoming Huge its str goes up to 26 (and then there are all the size modifiers to CMB, CMD, AC, Attack, etc). Am I correct?

Finally, I recall that a size increase from large to huge means +8 str, but I can’t find any confirmation in the books. Right now all I have is Animal Growth’s text and the fact that PCGen gives it a +8 str. Is there any standard reference for the matter?

How does it work the interaction between Magic jar and Find Steed?

Find steed spell describes:

…While mounted on your steed, you can make any spell you cast that targets only you also target your steed.

And, the spell Magic Jar is a "Self" range spell. How would this interaction work? Both your souls would go to the same container? Could you leave the container ending the spell on itself and keeping the steed on the container? Could you use two diferential container, one for you and one for the steed?

True Polymorph interaction with Find Steed

Find Steed (p240 PHB) States:

When the steed drops to 0 hit points, it disappears, leaving behind no physical form. You can also dismiss your steed at any time as an action, causing it to disappear. In either case, casting this spell again summons the same steed, restored to its hit point maximum.

If I were to cast True Polymorh (p283 PHB) on the steed and wait the full duration to make it permanent, would the steed return when summoned as the original steed or as the new creature it was polymorphed into?

Is this 9th-level spell Find Greatest Steed balanced with respect to other 9th-level spells?

Since the paladin gets the spells find steed and find greater steed, it only seemed natural to take this theme to its logical end: find greatest steed:

FIND GREATEST STEED
9th-level conjuration

Casting Time: 10 minutes
Range: 30 feet
Components: V, S
Duration: Instantaneous

You summon a spirit that assumes the form of the loyalest, majestic-est mount. Appearing in an unoccupied space within range, the spirit takes on a form you choose: a unicorn, a bulette, a felidar, or a nightmare. The creature has the statistics provided in the appropriate statblock for the chosen form, though it is a celestial, a fey, or a fiend (your choice) instead of its normal creature type. Additionally, if it has an Intelligence score of 7 or lower, its Intelligence becomes 8, and it gains the ability to understand one language of your choice that you speak.

You control the mount in combat. While the mount is within 1 mile of you, you can communicate with it telepathically. While mounted on it, you can make any spell you cast that targets only you also target the mount.

The mount disappears temporarily when it drops to 0 hit points or when you dismiss it as an action. Casting this spell again re-summons the bonded mount, with all its hit points restored and any conditions removed.

You can’t have more than one mount bonded by this spell or find steed at the same time. As an action, you can release a mount from its bond, causing it to disappear permanently.

Whenever the mount disappears, it leaves behind any objects it was wearing or carrying.

A mount summoned with this spell cannot take legendary actions. If it normally would have legendary actions, on its turn, it can use its action to take one of its legendary actions.

A paladin can cast this spell consuming two 5th-level spell slots, instead of one 9th-level spell slot.

This spell would appear only on the Paladin spell list, and could be prepared and cast by a paladin once the paladin was 19th-level. Additionally, this spell would be available to an 18th level Bard via magical secrets. I think this spell only being available to 18th level and higher characters is going to be enough to balance it. Compared to true polymorph, the effects here actually seem pretty modest for a 9th level spell; and for the paladin, casting is always going to be limited to once per long rest, as it uses up all of their highest level spell slots.

The mounts I have chosen range from CR 3 to CR 5. The original 2nd-level spell find steed mounts range from CR 1/8 to CR 1/2, and the now penultimate 4th-level spell find greater steed provides mounts ranging from CR 1 to CR 2. These two spells are given a comparative analysis in this answer. This CR 3-5 range seems like an appropriate increase in power, but as with both its predecessors, some of these greatest steeds will be less greatest than others. I’ve carefully chosen four creatures for this spell, I feel that each brings something unique to the table, even though one of them seems to be a head above the rest. Speaking of which…

The Unicorn (CR 5)

If I’m being totally honest, this spell could have been called find unicorniest steed. The unicorn is easily the best mount on the list. It is not the best damage dealer, not even close, but the utility and support the unicorn provides is unparalleled by other creatures on this list. It can cast pass without trace at will, and its ability healing touch is equivalent to a 2nd-level cure wounds twice a day.

The unicorn is the only creature on the list with legendary actions. I felt that giving the unicorn unbridled access to its legendary actions was too much. Additionally, its just easier to keep track of things when I’m not keeping up with my own turn, my mount’s turn, and legendary actions for my mount on other turns. Instead, the unicorn can opt to use one of its legendary actions on its turn. In particular the unicorn’s shimmering shield ability is quite good, and allows the unicorn to excel in its support role.

The Bulette (CR 5)

This guy is the bruiser of the group. At +7 to hit for 4d12+4 damage, the bulette’s bite attack hits like a truck, and AC 17 averaging 93 hp gives him respectable staying power. The bulette really gets interesting with his movement: burrow 40 ft. If you’re nostalgic about catching your first diglet in a cave outside of Vermilion City, the bulette is for you.

The Felidar (CR 5)

The felidar packs a similar punch to the bulette with identical AC and hitpoints, but the felidar is for the more psychically minded adventurer. The felidar has the ability to form a special bond with another creature, granting these benefits:

  • The felidar can sense the direction and distance to the bonded creature if they’re on the same plane of existence.

  • As an action, the felidar or the bonded creature can sense what the other sees and hears, during which time it loses its own sight and hearing. This effect lasts until the start of its next turn.

Similar combat prowess as the bulette, but has some interesting abilities that make the felidar an excellent scout and great insurance policy if his owner gets kidnapped.

The Nightmare (CR 3)

This goth version of the pegasus features an ability that makes it better than his winged celestial brother, earning him a spot on this list. For the most part, the nightmare is identical to the pegasus, which makes him probably the weakest choice on this list. But the nightmare has one ability the earns him his place here:

Ethereal Stride. The nightmare and up to three willing creatures within 5 feet of it magically enter the Ethereal Plane from the Material Plane, or vice versa.

This guy can disappear to the ethereal plane at will. And he can bring his three closest friends. The utility of this ability is limited only by your imagination and how annoyed your DM is that your flaming horse can walk through walls.

How do Polymorph and Find [Greater] Steed interact?

A bard has used magical secrets to select either Find Steed, or Find Greater Steed and has summoned a mount.

They then cast Polymorph on themselves and turn themselves into a T-Rex, because they are cool.

Find [Greater] Steed says

While mounted on it, you can make any spell you cast that targets only you also target the mount.

So Polymorph can affect both targets. However, since the steed is less than CR7 what happens?

  1. Both bard and steed both become a T-Rex
  2. Bard becomes a T-Rex and has to select a different form for the steed according to it’s CR
  3. The bard becomes a T-Rex but the spell fails on the steed
  4. The spell fails entirely because the steed isn’t a valid target to become a T-Rex
  5. The bard and steed become some kind of conjoined T-Rex abomination

If there is a difference between the spells, please use Find Greater Steed as the spell I care about, and assume the mount is a Griffon.

Can a mule benefit from a Phantom Steed?

The Phantom Steed spell creates a “Large quasi-real, horselike creature” that can be ridden by “you or a creature you choose”. “You decide the creature’s appearance” and it “uses the statistics for a riding horse.”

The rules for mounted combat state that “A willing creature that is at least one size larger than you and that has an appropriate anatomy can serve as a mount”

Can a mule ride on a phantom steed?

The conditions appear to be that (1) the Steed is willing (which I think we can assume from the nature of the spell), that (2) it is a size larger than the mule (Mules are Size Medium, Riding Horses are Size Large), and that it (3) has “appropriate anatomy”.

Now, obviously a real riding horse does not have an appropriate anatomy to be ridden by a mule.

But the phantom steed is not a riding horse. It is a “horselike creature”, whose appearance is decided by the spell’s caster.

So, RAW, does anything prevent the caster from stating that the steed created has its appearance including appropriate anatomy to be ridden by a mule, any other size medium or smaller quadruped, or other non-traditional riders?

What are the implications of the Find Familiar and Find Steed spells changing the creature type from Beast to Celestial/Fiend/Fey?

The Find Familiar and Find Steed spells both have this sentence describing the creature type of the summoned creature:

The [familiar/steed] has the statistics of the chosen form, though it is a celestial, fey, or fiend (your choice) instead of its normal type. (PHB 240)

I’m wondering what implication choosing celestial vs. fey vs. fiend for the summoned creature would have. I have three things I’m specifically interested in listed below.

  1. Is a familiar visible distinguishable as the type of spirit? For example:
    • A wizard with an allergy to cats conjures a hypoallergenic fey cat sprouting soft grass instead of fur
    • A tiefling paladin conjures a fiend warhorse steed to have a coat colour resembling a Nightmare
  2. Does the type have any effect on the familiar’s alignment? For example:
    • Inheriting the alignment from the creature’s stat block (unaligned in most cases) seems like the most direct option but would a celestial imp still be considered lawful evil?
    • If you command your fiend cat to sit on someone it may choose to painfully knead them whereas a celestial may be more relaxed and cuddly.
  3. Does changing a familiar to a new form allow it to change the type of spirit?

    If you cast this spell while you already have a familiar, you instead cause it to adopt a new form. Choose one of the forms from the above list. Your familiar transforms into the chosen creature.

    Adopting a new form sounds like it remains the same kind of entity. Does this mean to change the familiar from fiend to fey would require summoning a new spirit and therefore be treated like a new NPC?

These are grey areas in the rules, I am looking for official guidelines or other credible sources that can help me make an informed decision on how to rules these as a DM.