The cave was originally part of a tunnel system that Hades and a few other villains dug in hopes to find and reach the rumored weak spot in the barrier that keeps them all prisoner on the Isle of the Lost, so they could escape.
Description of Location
Hades' Cave has an entrance that has a skull with an old tire on one eye, and an old floaty on another, on it's nostril it has a key hole which Celia's key fits, it says "GET LOST!" on the top of the door, it also has a sign says “Beware of Dog!”
Role in the film
When Mal and her friends realize that they needed the power of Hades' Ember to combat the power of Maleficent's stolen sceptre, they returned to the Isle of the Lost so Mal and Celia could retrieve it from Hades' cave lair. Since Celia has been there a few times and knows her way around it, in when made her Mal's guide as she takes her to where Hades keeps his ember. When they got to the living area, the dogs barking is heard from the record player, and Hades is fast asleep on his comfy chair, Celia stops the record player causing Hades to wake up. All without knowing that Mal is Hades' estranged daughter, until she called him "Dad" when he caught Mal trying to take his Ember.