Do you need line of sight to cast spells on someone?

The rules on spellcasting contain the following section:

A Clear Path to the Target

To target something [with a spell], you must have a clear path to it, so it can’t be behind total cover. If you place an area of effect at a point that you can’t see and an obstruction, such as a wall, is between you and that point, the point of origin comes into being on the near side of that obstruction.

This section is not really clear to me. Should this mean that only non-transparent objects are a problem in targeting, or that you have to have line of sight and line of effect as well?

Also, can you prevent a wizard from casting spells by blinding her?

Is the Gloom Stalker’s Umbral Sight cancelled out by Devil’s Sight?

Most devil’s have the feature Devil’s Sight:

Devil’s Sight. Magical darkness doesn’t impede the [devil’s] darkvision.

The Gloom Stalker’s Umbral Sight says:

…You are also adept at evading creatures that rely on darkvision. While in darkness, you are invisible to any creature that relies on darkvision to see you in that darkness.

By my interpretation the Gloom Stalker will remain hidden even from a devil when in darkness, however my DM believes the devil will be able to see the Gloom Stalker with just their darkvision, as the effect is magical.

How does Detect Magic spell see a Warlock’s use of Devil’s Sight invocation?

I assume Eldritch Invocations are detected as magic, because they are magic:

In your study of occult lore, you have unearthed eldritch invocations, fragments of forbidden knowledge that imbue you with an abiding magical ability.

And Detect Magic spell detects magic like this:

For the duration, you sense the presence of magic within 30 feet of you. If you sense magic in this way, you can use your action to see a faint aura around any visible creature or object in the area that bears magic, and you learn its school of magic, if any.

What does Detect Magic reveal about a visible warlock with Devil’s Sight?

I guess there are two cases: the warlock actually "seeing normally in darkness" with the invocation benefit active, and the warlock being in bright light and invocation not doing anything. Let’s leave the murky details of Devil’s sight interaction with dim light and the warlock having or not having darkvision out of scope.

Can Tremorsense replace sight?

This question it prompted by the recommendation of Ryan C. Thompson in relation to their answer to an earlier question of mine.

Does tremorsense, allow a monster to effectively "see" other creatures or does it only indicate location?

The first option would have tremorsense function the same as blindsight with the exception that it doesn’t work on flying creatures and can "see" behind walls and other obstacles.
The latter option would function similarly to locating an invisible creature where you know the creature’s location but it remains unseen.

Does Tremorsense allow Attacks of Opportunity while Blinded?
Is a character unseen by tremorsense while jumping?
Blindsight and Tremorsense for a Blind PC?

How does trample work with hide and hide in plain sight?

Let’s say there is a treant shadowdancer who does not appreciate interlopers in its forest. Assuming it has enough hide skill to remain unseen, and as silly as it sounds, can this treant trample over everyone until they’re all dead while remaining unseen? Alternately, if the act of trampling negates the hide, can it use the shadowdancer’s hide in plain sight ability to simply hide again at the end of its movement since the hide skill states that hiding happens as part of movement?

Does the Flame Arrows spell spoil a Gloom Stalker Ranger’s Umbral Sight unseen benefit?

So this came up last night, and I ruled Yes, it does spoil it. I let the group know this was a one-time ruling as I wanted to post here to solicit an answer to set it in stone for the table.

It has been previously agreed a Gloom Stalker Ranger in darkness has advantage on every attack against creatures that require darkvision to see:

Does a Gloom Stalker making ranged attacks while in darkness have advantage on every attack?

I think the key point here surrounds this question:

Does pulling / drawing / shooting a Flame Arrows enchanted arrow emit light from the fire? At what point is the arrow or javelin "flaming", if at all? And if it’s prior to impact, does it spoil "darkness"? If it doesn’t flame, how does it cause 1d6 Fire damage?

There is nothing in the RAW for Flame Arrows which states that the arrows are flaming while drawn or in flight, but there is also nothing saying that aren’t. So I think this comes down to DM judgement call. I ruled that, like a Fire Arrow, it’s on fire before launched, and thus, makes the Ranger visible.

What do folks think, and why?

Does Devil’s Sight allow you to see in darkness as though it was normal light?

There are technically answers to this question both here, Sage Advice, a tweet by Jeremy Crawford, and this question here, but they still seem rather vague/non-descriptive enough.

Devil’s Sight (PHB Pg. 110)

You can see normally in darkness, both magical and nonmagical, to a distance of 120 feet.

But does "see normally" imply you see as though everything is naturally lit up by the sun; lit by a dim torch; still dark, but you can pick out clear images, shapes, and details; or something else entirely?

Does Devil’s Sight enable one to see into Hunger of Hadar?

The hunger of Hadar spell (PHB, p. 251) creates a black void of darkness, which cannot be penetrated by light. This means that no one can see in, and those inside cannot see at all, which is a handy way to damage and control enemies since they don’t know which way is out.

This also means that characters cannot easily attack those inside.

The Devil’s Sight eldritch invocation, however, enables a warlock to see normally in magical and non-magical darkness.

Does this enable the warlock to see into the area of blackness created by hunger of Hadar and attack creatures inside? Are there any other sight mechanics that allow someone to see in or out of the spell?

Does going outside of range or line of sight after casting a spell have any effect?

Does going outside the range of the spell or breaking line of sight between the caster and target break concentration or otherwise affect control over the spell?

For example, a Heat Metal spell has a range of 60 ft. If I cast the Heat Metal spell on a an armored foe, and that foe runs outside the range of the spell (60 ft), or around the corner, does that break concentration and end the spell? Heat Metal also allows you to use a bonus action to cause the 2d8 of damage — would exercising that option be prevented by being out of range or line of sight?