ToME Wiki
The Sacred Land of Mountains
The Sacred Land of Mountains is located in a large mountain range, and contains the remains of many old towns.
Equivalent to dungeon levels 45 to 70.
Terrain: Lots of mountains (big surprise!) with 1-square-wide passages and rooms. Non-mountainous terrain is often grassy. Single-square fountains.
Monsters: Usually at least one Thunderlord per level.
Boss: Trone, the Rebel Thunderlord
Discussion
MayLith: My priest (http://angband.oook.cz/ladder-show.php?id=2891) is immune to fire. Does Trone's time attack really hit for all that much?? She's got an additional 1165 HP from her symbiote.
Narchan: It looks like your player is a spellcaster, and I am not sure how much SP he gets from Toris Mejistos, but wearing the Anchor of Space Time reduces time damage(I think) so your character could swap it in for the fight. Also, since you have climbing you can run away easily if Trone is too much. You should be able to kill him or at least run away easily if he is too much.
ZV: I think that his time breath may cause 400-600 dmg. In addition, he can summon his kin, which often yields Marda, so a bad turns and you get double-teamed by two powerful uniques and some of their guard, which followed by time breath may cause untimely demise. In case of this character, I advise extreme caution when approaching Trone, but you'll be able to handle him. Just be paranoic, it always helps in roguelikes 
MayLith: The Anchor provides time resistance. Climbing sounds like a VERY good idea. ZV: Oh, goodie.... Maybe I'll climb down through LoM instead...ya think I'll find Marda first that way?
ZV: There's a little chance that you'll find Marda earlier. Though you can get some dungeon towns with nice stuff and all
MayLith: One dungeon town. No Marda... ergh. By the way, I posted a cmovie of my (very quick) encounter with Trone over on oook.