Ancalagon
Dusty Dragon
It's 300 feet down, but you've probably noticed that the Lair is wider than the Rift. I imagine that it opens up at about 200 feet, giving the Lair a 100-foot ceiling with the rift as a "skylight". So the last 40 feet of your first round's climb will be upside down on the ceiling, which is tricky but not impossible for Udit.
He has good boots.
(I'd probably ask for two rolls, with the first dc being pretty easy and the second being a little tougher. Falling wouldn't happen unless you botch it, you'd just get "stuck" for awhile).
Otherwise, it would be 100 feet down from there. He should be able to get to a Lair Wall in that 40 feet, though. Then the following round he could pretty easily slide down to the ground (IE easy DC).
Oh I could do that... or I could go MISSION IMPOSSIBLE STYLE. Check my equipment. I have 100 feet of rope



