So i've been thinking lately about making a campaign that starts as three or four parts, with different groups of players playing each part. After all the parts have been completed, all the players then have to come together to defeat the puppet master bad guy.

Basically for the story I'm thinking some ancient powerful being, not god, not demon, not devil, something else that I'm working on, and centuries ago he tried to destroy the plane the campaign takes place on. He failed, and then realized that he needed help to do it. So eventually, he escapes from his eternal prison and recruits some evil chromatic dragons, a mind flayer colony (or a beholder, haven't decided yet), a lich, and a Demon Lord (of my own creation) to act as various parts of his grand plan. The lich supplies him with an infinite army, the dragons act as enforcers and the strongest members of his army, the mind flayers/beholder would be his research division, and the Demon Lord would possibly be some kind of scapegoat so no one knows he's active again while also supplying extra soldiers and whatnot. Of course, they wouldn't help him for nothing, so he promises them various things. For the dragons, the grandest of treasures. The mind flayers, a sliver of their kingdom (or the beholder to be the true epitome of his race). For the lich to live beyond even the death of the multiverse. And for the Demon Lord, absolute control over the Abyss.

So each group of players would encounter one of those "sides", the dragons, mind flayers/beholder, lich, and Demon Lord. After striking a blow to each "side", they would retreat back to the BBEG after revealing that there is a grander plan in motion. This is when all the groups would come together to stop this grander plan.

So any thoughts on this whole thing? To ambitious, maybe some plot holes, story elements that might add more flavor? I'll take anything really, and anything would be appreciated.