Another thing to consider is the "comic book villain" approach in the instance the villain does die per game mechanics. The great villains never truly die, even when they technically DO die. They always come back eventually in some way, shape, or form.
Some standard tropes for this are:
It was a clone, a double, a cyborg robot, a magical construct, their twin brother, ect.
It was the villains body, but his mind is still in tact in some way. Magically projecting the entirety of his mind into someone else (this would make for a fantastic death curse IMO), digitally uploaded his brain into the form of an AI, ect.
The villain had kin. A son who's the spitting image of the villain, a parent who's even more of a terror than the guy you killed, a vengeful spouse, ect.
The villain is so powerful/clever that he managed to cheat death. He had minions who followed the villains "plan Z" and brought him back to life, the villain is so ruthless and tenacious that he actually managed to find a way out of whatever afterlife he was in and come back to the realm of the living, having a deal with some dark power that prevents him from dying before a set time/circumstance, ect.
The bumbling clueless person who accidentally brings them back. Reading from some old creepy book some teenagers find in a cabin, a would be warlock accidentally brings back the past/alternate universe version of the villain into the present day.
Some of these examples obviously don't work in the Dersdenverse, and I wouldn't use these as a crutch if the players keep killing your villain over and over again. If your intended recurring villains keep dying, then they probably aren't very good villains in the first place. So you as a GM need to be a good villain, always have a backup plan!