The Ancient Lair is a place you can get access to by crafting a Relic of Thunder at a workbench. Once you craft it and use it, you will be taken to an alternate version of the Worm Pit. If you decide to leave, you can still come back. It will show a red skull as the icon on the map. The difference is that this worm is level 35 and is a lightning elemental worm and the other one was an earth elemental worm. Nothing changed in the fight except it has some new drops. One of the drops is a Grape Larva Pet. So if you want to fight this worm, then train and get ready for a thunderous battle!