Vecna utters a dread word to interrupt a creature he can see that is casting a spell.
If the spell is 4th level or lower, it fails and has no effect. If the spell is 5th level or higher, Vecna makes an Intelligence check (DC 10 + the spell's level). On a success, the spell fails and has no effect.
Whatever the spell's level, the caster takes 10 (3d6) psychic damage if the spell fails.