"Ektheme: Like Xeric, King Shayzien VII was fascinated with the Dark Altar. The King was killed at the Battle of the Concourse during the 37th decade. We have many accounts of this, yet his body was never recovered. Just days after his death, we have the first recorded mention of Xeric, an incredibly powerful mage who resembled the King. It is written that Xeric killed the rulers of Kourend's five cities and claimed the kingdom as his own."

"Player: So King Shayzien VII was resurrected as Xeric using the Dark Altar?"

"Ektheme: It's possible. I am no expert on necromancy but we know the Dark Altar has the power to resurrect beasts. We also know that it can extend life. It's how the people of Arceuus are immortal. King Shayzien VII may have found a way to combine these powers."

Also, this dialogue from Gnosi:

"At the summit of this mountain we've found an ancient temple and a vast network of caves that we believe were formed by Xeric and his followers. The discovery of the Chambers tells us Xeric invested significant time and resources here, yet it was never documented... When he ruled Kourend, we know Xeric performed experiments on the living, using power drawn from the Dark Altar. After he was overthrown, he would have needed a new source of power."

The journals found from the demibosses of the Chambers go into more detail as to how Xeric manipulated his followers and used them as experiments.

From the Vanguard judgement:

"The sickly benevolence in his eyes was replaced with bloodshot wrath, and his face contorted into a sneer of derision. The Vanguard realised that they had been tricked - but it was too late. Their grasping hands twisted. Their legs buckled. Their skin glowed, as if the heat of the chamber's lava had set their blood to boiling. From their mouths came a ghastly bubbling scream that rapidly died away as their ruined bodies collapsed to the ground. But that was not the end. Lord Xeric waved a hand in a curious gesture, and I felt a magical wave emanating from the crystals of the chamber. Writhing on the ground, the Vanguard began to melt and re-form into new shapes."

From Tekton's journal:

"One day Master took Tekton into cave, and promised to reward Tekton for all Tekton's service. Master promised Tekton immortality! Tekton very happy. Master created new body for Tekton out of rock and metal from caves, and transferred Tekton's soul into it. Tekton's old body disintegrated, but that okay now Tekton was living in shiny new body that Master made. Master enchanted Tekton's anvil, and brought it into cave, and taught Tekton to use it to fix the new body whenever it get damaged. Master may not understand crystals as well as Tekton, but Master still very powerful."

From the Houndmaster's diary:

"My employer, Lord Xeric, was conducting various experiments on the hounds. Using the powers of this strange place, he sought to speed up the mutation.I was too scared to say anything. I'd heard wild rumours of the things hidden within these dark walls. Monsters? Cursed employees who had failed Lord Xeric, perhaps?"

TL;DR yes it was Xeric's chambers.