If you fight against witches, there is a chance to get afflicted with a dark curse. Go talk to Farengar and he will tell you the only cure, is to share your wife 10 times.   By default, Serana is marked as wife, but you can set any NPC you want, wife or not. Use the debug menu. Activate it with console command set debugcuckold to 1 You can disable the menu by setting the value to 0, the same way. You can also activate/deactivate the curse at any time. If you want to