The Vale of Thaena is a very rugged valley of the Inghild Peaks. Scarred with cracks and ravines, it has an out-of-place evergreen tropical forest. Steam is always rising from somewhere; this the cooling mechanism around a demon giant imprisoned under the valley floor.
In the Demon Spawn War (8972 GE - 9493 GE), a son was born to Surtur and a cunning demoness; a succubus named Ezryn in service to Rilthang. Charmed during the copulation, Surtur wanted nothing to do with a son tainted by demon blood. Named Thaena by his mother, their son's bout of anger and rage resulted in him becoming ever hotter. Reports of the time say that his rage brought on such heat that even fiery beings like Surtur and Kossuth could feel it. Warm to them, it was brutally hot to others, some say in fits of rage, Thaena was like a miniature star, burning the land, sucking in all moisture around him. A danger to both sides, in one of the super rare moments of cooperation, the Covenant and the demon lord Ogtha put aside the war until the matter of Thaena's rampages were put to rest. Willing to sacrifice an army to lure their target into a trap, Ogtha sent two demon divisions into what would become known as the Vale of Thaena. The Covenant, thanks to the Creation War (1486 DE - 8777 DE) and its primordial dreadnoughts, were masters at building magical prisons. They built a magical trap that snared Thaena, then sunk him into the ground while great quantities of water were showered upon him magically and by summoning a spring storm. Once in place, Thaena would be forever cooled by the natural streams and precipitation of the land around. Quite a bit of work went into shaping the area beneath the surface, guiding the water to where they wanted it to go. Once this work was done, the area was backfilled, then laden with magical traps so deadly that even Lukoon would give pause.
In the Horgon Era, detecting great magical power in this valley, the dragon Nveryioth settled down in the area above Thaena's prison. There he and his followers built the great castle Khilgand.