|The Banshee Tree by Jackie Musto|
As with characters who fail Powers Checks and become Domain Lords, we viewed this as a point of no return where things got too crazy and moved too far away from standard adventures to work. So the character had to become an NPC. This makes sense to a degree but lately I've been thinking that it is unnecessary, that is is okay for the character to continue as a PC, at least if the group and the GM are willing to go forward. As long as the transformation leads to a place that is still workable in a campaign, I think there is a lot of fun to be had there.
This end stage of evil could just be a late stage, it need not be the final stage. I would liken it to the domain management level of a standard D&D campaign. In that case the focus shifts from adventuring to managing followers and politics.
It is definitely possible to continue running a game after one of the characters Grims, in the case of Sertorius, or becomes a Domain Lord, in the case of Ravenloft. I saw this firsthand with Sertorius where we allowed characters to go well beyond where they normally would still be considered PCs. The trick is to figure out how to continue with "the party" after one or more of their members has made such a drastic transition. I have a couple of solutions here.
The first is collecting transformation: the whole party is turned together into something new. They share in the spoils and the punishments. While retaining their individuality they all are a part of this new domain in their own way.
The second allows for disparity and imbalance. In this approach only the character who transforms makes a true apotheosis into something greater, but the other characters ride his coattails and are drawn in as well. They may form mental bonds with the transformed character or become his minions with limited abilities and powers.
The important thing with both these approaches is to make this an interesting venture for the whole party to participate in. It requires a certain amount of buy-in from all the players so you want to make sure the transition is something everyone is on board with (it is just such a radical departure from where the campaign began that you want to make sure continuing is worth your time and effort).