The Warlock Invocation Fiendish Vigor allows the Warlock to cast False Life without expending a spell slot or using material components.
https://rpg.stackexchange.com/questions/106415/should-i-just-assume-that-a-warlock-with-the-fiendish-vigor-invocation-starts-ev https://www.reddit.com/r/dndnext/comments/8rc7rj/false_life_warlock_invocation/ https://www.dndbeyond.com/forums/class-forums/warlock/17166-fiendish-vigor-rule
https://explorednd.com/gameplay/false-life-5e-guide/
This gives Warlock players an chance to break the 4th wall and continuously re-cast False Life outside of combat to ensure they get 8 temporary hit points.
There are two approachs to handling this Invocation:
- Always give the Warlock 8 temporary hit points.
- Only allow one cast/roll of the temporary hit points.
I originally went with the first option and didn't make the Warlock roll at all since multiple casts made it deterministic but realized much later the character shouldn't know how effective the spell is -- they probably can't tell the difference between 5 hit points and 8 hit points.
As for metagaming, players shouln't know monster hit points and they aren't supposed to know another player's hit points; the only reason they know their own hit points is because their responsible for managing their own hit points to play the game. If a player needs healing, they shouldn't be saying how many hit points they have (or need)
https://www.reddit.com/r/DnD/comments/8insie/should_players_be_allowed_to_know_hit_point/