A "Drachenmensch" - litarally translated as "dragon-human", I don't know how to translate it better - named Seldrakon had once founded a dragon cult in the mage wars - the "Magierkriege" of the LMK times. He was dead, apparingly, and his followers had worshipped his mummy.
But he was not dead, awoke recently - after the "Drakensang 1" story - and had led his worshippers into the Kosch to Avestrue.
The cult followers were always wearing ed robes (for fire) and golden masks - and ONE golden mask was fitting into the face of that "dragon-human", and it had magic powers (not specified in the text, I think. I'm browsing through it right now).
Through the deeds of the adventurers in Drakensang 1, this cult was destroyed - apart from one last high priest who was left in Avestrue to guard the mask. Brutochor is his name.
Goblins found the mask when they were beginning to use the caves as their new "home", and the Goblin Shaman (female, of course, as typical in Goblin culture was upset, kidnapped Binga Ackerboldt, and plan to sacrifice her to keep that "evil ghost" at bay - but that "evil ghost" is no-one else than Brutochor, whose presence she is able to sense when she takes on that particular golden mask, the book says.
In the depth of the caves - which look more and more like made by humans - Brutochor is planning to get those invaders of the caves and the crypt of the Mantra'Kim out - the invaders are of course the adventurers. The golden masks helps them to get over 2 guardian-ghosts.
Brutochor is planning to summon a Mantra'Kor for this purpose - which is somewhere there in the depth.
The book says that this Mantra'Kor was given to Seldrakon personally by Ardakor.
His succeeds in summoning this beast, and gives orders to it.
But - if Brutochor is defeated - which is in fact the only chance of survival for the adventurers - then this Mantra'Kor is going rampant, destroying EVERYTHING. Until the crypt collapses, and thus kills the Mantra'Kor (and BRutochor, if he still lives, as well).
In the aftermath, Archon Megalon appears, saying that he is interested in that particular mask …
The Mantra'Kor is described as "four steps high", being as heavy as a Troll, having leather-like wings, and walking on two legs. It also wars a golden mask - a very different one, though. The cover of the book depicts it - and no other look is known. There seeminggly just wasn't drawn more of it.
Wiki Aventuricca contains a link to some piece of fan art, giving this monstrum a possible look :
www.wiki-aventurica.de/wiki/Mantra'Kor