Does the Mind Spike spell end if the target foes out of range?

Mind Spike says

On a failed save, you also always know the target’s location until the spell ends, but only while the two of you are on the same plane of existence.

DnD Beyond lists this having a range/area of 60ft.

Does this mean if my target teleports (remaining on this plane of existence) away I stop knowing where they are outside of the 60ft? Or is that the initial range?

Can charmed foes be considered “allies”?

I’m researching specifically the Arcana Domain cleric feature, Spell Breaker (SCAG, p. 126):

Starting at 6th level, when you restore hit points to an ally with a spell of 1st level or higher, you can also end one spell of your choice on that creature. The level of the spell you end must be equal to or lower than the level of the spell slot you use to cast the healing spell.

If I charm an enemy and then cast a healing spell on them, would I be able to remove a beneficial spell on them (like crown of stars or stoneskin)?

There may be a circumstance where I didn’t prepare dispel magic or have a level 3 spell slot to use for it, so I’d like to know what my options are.

If a charmed enemy is considered an ally, please include whether that extends to enemies charmed not by you, but by your allies as well.

Are the Tiefling subraces from Mordenkainen’s Tome of Foes specific to a particular campaign set?

Are the Tiefling subraces from Mordenkainen’s Tome of Foes specific to a particular campaign set?

In Mordenkainen’s Tome of Foes (MTF) there is a section on Tiefling Subraces (p.21), which elaborates on how the information about a Tiefling’s ability score increase and the Infernal Legacy contained in the Player’s Handbook relates to Asmodeus. Let’s say the “default Tiefling” prior to MTF.

Under Tieflings Subraces there is a variety of Subrace Traits for e.g. Tieflings associated with Baalzebul or Fierna.

It says this is a DM’s option to include other Tiefling subraces, but I was wondering: Is this intended specific to a particular Campaign Set, or it’s meant to be a global change?

How to prevent a single-element caster from being useless against immune foes?

TL;DR: How can I prevent a full single-element caster from becoming almost useless against foes immune to his element of choice, without making him overpowered? To make the question more general, I wrote “caster”, but in this case it’s a Sorcerer.

I’m DMing a newly-started D&D 5E campaign with 5 new-to-the-game players, one of which (“Mark”) chose to be a Sorcerer. Mark has a particular love for electricity in general, so he thought of a character obsessed with lightning to the point where he refuses to study (therefore to know) any non-lightning spell, thus all his attack spells will be limited to those dealing lighting damage (e.g., Lightning Bolt, Shocking Grasp). He doesn’t want to change other damage types into lightning (i.e., no lightning Fireballs), he simply won’t learn any attack spell that isn’t already based on lightning and doesn’t deal lightning damage. When there’s no choice, he will opt for utility spells. Even further, he will never do any attack that deals any other damage type except lightning.

Now, that’s great, I think there’s good roleplaying potential for such a character. However, as a DM, I’m concerned about his combat usefulness against foes immune to lightning; I guess this comes from my inexperience. Yes, he could have a few utility spells, but I don’t think they make up for the complete loss of damage. I warned Mark that enemies with resistance to lightning might show up, prompting him to choose Elemental Adept (Lightning) as a feat, which he understandably found fitting for the character anyway. However, I added that enemies immune to lightning might show up as well and Elemental Adept won’t be of any help in such cases.

  • Mark said he doesn’t want to give up the lightning-only trait of his character and would like to be helped figuring out something.

  • He is level 2, as the whole team is. I haven’t considered magic items yet,

I want to find a way for his character to still be useful against lightning-immune enemies (e.g., the Kraken, which I plan to unleash further down the story), without breaking him in terms of power. I thought of different options, for example:
Elemental Adept ignores simple resistance and reduces immunity to resistance; at the cost of 1 Sorcery Point, he can choose to turn the damage type of lightning spells into force, but as this is my first time dealing with such a case, I’d like to hear some opinions and advices.

What is the winged creature on the back of the Mordenkainen’s Tome of Foes book?

On the back of the Mordenkainen’s Tome of Foes book, there is a small creature with wings (I don’t know how to crop images, so here’s the entire image):

enter image description here

What is this creature? It doesn’t match any of the creatures in any of the monster manuals. It kind of looks like an imp, but it clearly isn’t one, and my only other guess is that maybe it’s some kind of homunculus?

Quest Flaw of I refuse to hide from My foes [on hold]

I was playing The Artifact as a Rogue and in Game I was Given the Gift of Prophacy which comes with a ability you can use twice before it vanishes but has the caviot of altering appearance and adding the flaw of I refuse to hide from My Foes, both of which it says nothing about going away. So does that mean I can never hide when I perceive hostility? What about cover? Seems like a absolutely ridiculous quest flaw if not even a remove curse can get rid of it.