So, one of my players might get his Druid killed soon and then I thought "why not make the Druid a recurring villain?". To justify the turn to evil I decided that the Druid would be corrupted by an evil force. I am a little unsure of which evil force should corrupt the Druid. Demons and Devils are obvious candidates, Fey is also a good choice since I'm running Kingmaker.
The problem is that I don't know what flavor of corrupted I should go with and I need a reason for why I can have the Druid return from apparent death. The Druid also needs a motivation like "You betrayed me, and for that I will destroy all that you hold dear." or "Your kingdom will ruin the land. I will tear it all down and let the nature takes it's rightful place." to match the change.
Any suggestion for how I should do this?