You Cannot Pass!
Instant
Destroy target creature that blocked or was blocked by a legendary creature this turn.
It raised the whip, and the thongs whined and cracked. Fire came from its nostrils. But Gandalf stood firm.
Leonardo Borazio
Rulings
You Cannot Pass! only cares that the second creature was a legendary creature at the moment it blocked or became blocked by the target creature. If that legendary creature has become a nonlegendary creature (perhaps because it's no longer your Ring-bearer) or left the battlefield, You Cannot Pass! can still target the first creature.
(wotc • 2023-06-16)
You Cannot Pass! can target a creature that's currently blocking or being blocked by a legendary creature, or one that blocked or was blocked by a legendary creature earlier in the turn.
(wotc • 2023-06-16)
Rulings
You Cannot Pass! only cares that the second creature was a legendary creature at the moment it blocked or became blocked by the target creature. If that legendary creature has become a nonlegendary creature (perhaps because it's no longer your Ring-bearer) or left the battlefield, You Cannot Pass! can still target the first creature.
(wotc • 2023-06-16)
You Cannot Pass! can target a creature that's currently blocking or being blocked by a legendary creature, or one that blocked or was blocked by a legendary creature earlier in the turn.
(wotc • 2023-06-16)