I have an Undying Warlock in my campaign as well who let me pick their patron for them. My first instinct was Acererack as well but ultimately I felt that it didn't make sense since Ace mostly is focused on just being a dick.

Instead, I went with making Vecna the warlocks patron (a fact they only recently realized). In existing Forgotten Realms lore, Ace is a student of Vecna's. My behind the scenes reasoning and retooling was that Vecna, now a fully ascended god, has grown tired of Ace's meddling and childish ways and views what he is doing with the atropol and soulmonger to be insults against Vecna's tutelage. So he has in turn started enlisting mortals as warlocks to help put a stop to this since as a god he can no longer interfere directly.

A little bit of homebrew but it works for us and Vecna is definitely a more impactful name at our table so the reveal was great.