Your players have a great many options, given that you are open to homebrew material:
- They should seek out a Couatl, and undertake a quest to receive it's shielded mind trait
- They could ally themselves with a Flumph and stay very very close to them, so as to benefit from their telepathic shroud. Perhaps, if any of their group is psionic, they could learn to manifest such a shroud themselves.
- They could acquire the aid of a rakshasa, and willingly invoke its curse upon themselves, filling their every thought with horrible images and dreams, and thus discouraging the use of telepathy against them.
- They could start a weekly poker game with an androsphinx and his consort, and so acquire a measure of their inscrutability.
(Basically, any time you're going 'I wanna do a thing/provide my players an opportunity to become able to do a thing' and it's not easily done via spells/magic items, check out the monster manual. Most monster traits are presented in such a way as to make it fictionally reasonable for players to gain access to that trait via some sort of interaction with the monster, be that murdering it and harvesting parts of its corpse or training under it for an extended time period or performing quests for some divine patron or eldritch being or another)
Additionally, in previous editions spell research rules existed, and the ones from AD&D 2e are pretty consistent with the 5e ethos. For some reason (probably game balance-- spell creation in D&D has always been hugely difficult for GMs to properly adjudicate) 5e lacks spell creation rules, but you could certainly import or modify ones from elsewhere. A spell similar to 3.5's False Vision, which has no 5e counterpart yet is not particularly problematic would be a reasonable addition.
I have added False Vision as a 4th level illusion spell with the ritual tag for a wizard in 5e before. I had it require concentration, last up to 24 hours, and require an action to alter the image viewed. I also added a 6th level version that took an hour to cast and didn't require concentration on a round as long as the caster used no movement that round. I changed the material component to a focus component for each. Both worked fine.
Alternatively, a variant on one of the spells that protects or disguises against detection spells could be a decent starting place; they almost do what you want as-is, it would seem.
It depends on what the enemy does in reaction to being hit by fireball, which is situational and up to the GM
The fireball spell description doesn't say that it reveals enemies, but on the other hand, it doesn't preclude that immediately subsequent events would reveal them.
For example, the GM might deem that a hidden or invisible enemy would cry out in pain, thereby giving an auditory indication of their presence (but perhaps not pinpointing their exact location -- unless you win on a passive Perception check). But alternatively, the GM might deem that the enemy has the toughness of will to remain silent.
Also consider that even if fireball "misses" (does zero damage, e.g. if a rogue prevails in using his evasion feature), a similar thing could happen. Doing zero damage to a hidden and evasive rogue in the area requires them to have made a Dex save, which implies that the enemy jumped or dove out of the way in the nick of time. The GM might deem that a hidden creature diving away from a fireball, is thus visibly revealed. Or the GM might deem that the hidden creature has a large enough hiding space in which to jump away and still remain hidden.
As another example, suppose an NPC caster is making herself invisible by casting a spell; she might lose concentration as a result of taking damage from fireball, and thus become visible. But of course, she might not lose concentration, and remain unseen. So again, the outcome cannot be known until we see what happens in the next instant after the fireball's explosion.
Nothing in the spell's description entitles us to infer that enemies are automatically revealed (along the lines of the accepted answer to this question), but in some situations, the GM could reasonably determine that what enemies do in the situation does reveal their general presence, or even which space they are in. And likewise, I would think, for most any AoE spell effect.
Best Answer
Mind blank foils legend lore
Mind blank states that:
Legend lore states that:
Legend lore is clearly a spell used to gain information and so it is foiled by mind blank.
Note that mind blank doesn't foil attempts to gain information from its target, it foils attempts to gain information about its target, which means that:
So long as the information is about mind blank's target, then attempts to gain it via spells (or similar effects) are foiled.
Whether nondetection prevents legend lore is unclear
Nondetection states that:
Legend lore is certainly divination magic, but does it target the named/described person/place/object?
"Target" is ambigous in 5th edition and in the case of legend lore, I do not think a solid case can be made either way.
When you cast legend lore you are choosing something by naming or describing it and this is similar to other spells where you choose your target(s). However, other spells subject their targets to an effect, whereas legend lore doesn't: you learn information about something, but nothing happens to that thing.