This is just an aside, but I think Acererak was basically doomed to fail from the start. There are some really big names on the list of Liches who would not be happy with his plans, and some of them are very capable of beating Acererak if it came down to the wire. Being secretly guided to stop him by one of those liches is a great plot hook.In fact, as written one major Forgotten Realms lich is indeed very interested and has agents very actively doing exactly that.
Similarly, since Acererak is doomed to failure from the start, I like to imagine the whole thing is a big diversion for something else Acererak is planning to do.