I still think that planning for a villain to escape alive is risky unless the fight is totally rigged.

You want the scene to play out like a classic horror movie bad guy, so you have to rig it like a movie script without making it seem rigged. I would run it that Alan has more than 18 hp, but as soon as he takes 18 pts of damage or more, one of his various escape mechanisms triggers. I would think of five or six before hand, and try to make them fun and creepy. A 3 use ring of Meld Into Stone, crude slide tunnels (where after Alan goes down, spear traps trigger, waiting to impale pursuers), he dumps a bag of live rattlesnakes at your feet and then runs away, he triggers a cave in between him and the party that spreads, so they have to retreat. Also, every time he comes back his AC is a little better and his attacks are a little stronger as he slowly collects all the murderer contingency equipment he has hidden all throughout his burrow.

Although if I were running this, I might just have him attack again even if the players killed him.

He’s basically this thing.