He's just like "Hellooooo! How arrrrre you? It's so nice to see people exploring, seeing what there is to see. You must come in and tell me about allllll of your adventures out in this vast and bulbous worrrld."
It's the slowest voice I can do. Like James Earl Jones in the This Is The Book That Got Bubba Cooked commercial, only smilier. And oilier.
He doesn't attack or anything. The players have no idea what his saves or armor class or hit points might be. He just wants them hang out with them and eat fat feasts and drink red drinks. He was imprisoned for thousands of years until the party showed up and freed him and so right now he just wants to, like, hear about how humans finally figured out aqueducts and post-and-lintel architecture and shit.
If they try to leave the dungeon, he's all "No, no, you should stay, you should stayyyy" and if they walk off then he casts Slow. And if they still keep trying he casts Slow again. And he's a demon of Sloth (all demons are of a Deadly Sin--did you know that?) so the Slows stack.
The players are fucking terrified of Anaxorchas. These people will and have fought 300 guys no problem. They haven't even considered fighting Anaxorchas, they just sort of laugh nervously at his jokes and play for time. He's been around for like 3 or 4 sessions now.