You can only reach Xardas in Chapter 4, after Saturas sends you to find him. His tower is barred by a talking demon that demands three golem hearts (ice, fire and stone) before it will let you pass; only then can you enter and speak with him. He trades magic items and teaches Orcish.
Xardas' tower in the orc territory — Chapter 4: A thirteenth, dark-magic mage who lives in a tower out in orc land. A talking demon guards the door and will only let you in once you bring it the hearts of an ice golem, a fire golem and a stone golem.
Xardas' tower / Sleeper Temple — Chapter 5: Becomes the key ally for bringing down the Sleeper: he needs the five gems from inside the Sleeper Temple and reworks the sword Uriziel so it can be recharged and used against the Sleeper.
Quests Xardas takes part in across Gothic 1 Remake.
10,000
You: Teach me Orcish.