A little OT - Unless I'm missing something in the stat block, Vecna can cast his "Rotten Fate" ability without limits. IMO, that is OP.
"Rotten Fate. Vecna causes necrotic magic to engulf one creature he can see within 120 feet of himself. The target must make a DC 22 Constitution saving throw, taking 96 (8d8 + 60) necrotic damage on a failed save, or half as much damage on a successful one. A Humanoid killed by this magic rises as a
zombie (see the
Monster Manual) at the start of Vecna’s next turn and acts immediately after Vecna in the initiative order. The
zombie is under Vecna’s control."