It raised the whip, and the thongs whined and cracked. Fire came from its nostrils. But Gandalf stood firm.

You Cannot Pass!

Destroy target creature that blocked or was blocked by a legendary creature this turn.

The Lord of the Rings: Tales of Middle-earth

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.

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.