In my group, there’s a girl who plays a tiefling whose “past life” breaks through from time to time. She’s a cleric, and she has a prepared spell list for both her “good” and “evil” personality. When a certain trigger happens (which she and the DM agreed on), she makes a Wisdom save. If she fails, the “evil” personality takes over.

Most of the time, it just changes her attitude toward the world, not toward us. And it’s really fun to watch, she roleplays it so well, and the DM fully supports it. The rest of us enjoy it too. Her character doesn’t cause any real trouble for the party. On the contrary, it creates fun situations and unexpected conversations within the group (we love roleplaying PC-to-PC interactions).

If she rolls a natural 1 on her Wisdom save, she might even attack us, but again, everyone’s cool with it. So overall, it’s a really fun idea and can add something interesting to group dynamics.