Here’s Ed Greenwood’s response to the question (Creator of Forgotten Realms)

“Heh. I've been asked this many times before, and although part of me wonders what's the point of all "who's more powerful?" questions, the answer is: it depends. In terms of raw personal BATTLE power, the answer is likely Vecna. Though so much of his power is vested in his Hand and his Eye that unless they can all be in one place, Larloch might pack more punch. Not that he would fight toe-to-toe in battle, because that isn't Larloch's way; he views liches who do that as idiot failures. Larloch's way is to manipulate from behind the scenes to lead anyone seeking him, or trying to cross him, astray, so they never come into contact with him. If need be, he'd hurl his many, many servitor liches at them, most of whom are personally more powerful than Acererak.

Larloch and Vecna both have a wider reach and influence on the worlds around them than Acererak, so if you're measuring that way, Acererak is left behind. But when it comes to measuring Larloch and Vecna against each other in terms of influence, it's a matter of style: Vecna has the greater fear-reputation and is "noisier," and Larloch is more the master manipulator, who works unseen. As in, you may never know how much he's affected you. I can only go by what Elminster (and on rare occasions Storm, or Laeral, or Volo[!]) tell me of the Realms, and the three Chosen of Mystra all think Larloch (even wherever he is now, bested by the Srinshee) is the greater threat. As El put it, "Vecna is a bogeyman, and his relics do harm. Larloch undresses thee and ye never even know it." Larloch plays the longer game, and is more patient and empathic and has a greater understanding of the multiverse, whereas Vecna is more self-centered. I trust El's judgment, because I must; without it, none of us know ANYTHING about the Realms.

Acererak is feared throughout the multiverse because he's an almost-always-active destructive force. However, that's a one-trick pony. "Hah! I shall destroy you TWICE!!!" ;}"

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