Plot Dependant

There’s a common joke that Dungeon Masters should never introduce an NPC or villain they aren’t willing to have killed. Because even if they’re ten levels higher than the party, the PCs will somehow stunlock them and drop a moon on them. That’s when they’ll break out even forgotten magical item and boon you’ve even given them but they’ve been sitting on an hoarding for years.

The best recurring villains aren’t planned. They’re the survivors. The ones that somehow don’t die or get slaughtered. The ones that somehow, despite all odds, just manage to run away. They suddenly become more important solely because the players hate them for daring to live!