How do you handle loot from enemies with mighty or evil weapons?
For example a Flind (a Gnoll from Volo's) wields a mighty evil flail imbued with magic from Yeenoghu. Do you let the players have the flail and use it? Or do you prevent them from having it some way? In that case how?
Here's what I'd do if I wanted to give the a powerful magic item: The
three-headed flail of Yeenoghu is a very rare magic weapon requiring attunement. It is heavy two-handed martial weapon that deals 1d10 bludgeoning damage.
While attuned, the wielder suffers madness of Yeenoghu (described in
Out of the Abyss & I think online) and suffers whatever curse you feel is apropos (e.g. like a
sword of vengeance).
When attacking with it you can only use as many heads of the flail as you have attacks; for example, a 5th level barbarian, fighter, monk, paladin, or ranger with Extra Attack could use two of the heads, while an 11th level fighter could use all three heads.
Head of madness. On a hit, the target must make a DC 16 Wisdom saving throw. On a failed save, the target must make a melee attack against a random target within its reach on its next turn. If it has no targets within its reach even after moving, it loses its action on that turn.
Head of pain. On a hit, the target takes an additional 2d10 psychic damage.
Head of paralysis. On a hit, the target must succeed on a DC 16 Constitution saving throw or be paralyzed until the end of its next turn.