After finding the Uriziel sword and powering it up, you'll start Chapter 6 of Gothic 1 Remake, The Sleeper's Halls. In this chapter, you will finally face the Sleeper and lift the magic barrier in the colony.
First, head back to Xardas after powering up the Uriziel sword. He'll give you a few hints as to what to do next. You must prepare for the final confrontation up ahead, so make sure to stock up on food and potions.
Head back to the Orc Enclave and into the Sleeper's Temple entrance. The Orcs will inform you that Kor Calom has entered the temple and is trying to bring the Sleeper into this world, so you must act quickly to stop him.
Inside the temple, you'll find that the halls now brim with stronger enemies than before, including several Templars, High Temple Guards, and Skeleton Mages. You can choose to fight them, but it's better to run past them, as the way forward is clear - You don't need to solve any of the puzzles again.
Sprint to the bridge leading to the magical barrier, which has three Templars defending it. Go up the stairs, and you'll find Xardas, who has been weakened by the Sleeper's magic and will ask you to progress further ahead.
Break the magic barrier with Uriziel and enter the final room in the temple, with several more templars inside. You can find Mad Kor Calom waiting further ahead, and finally reach the Sleeper at the end of the temple.
Fighting the Sleeper
The Sleeper is not a conventional boss fight. He will instead make you face several past companions turned enemies, one after the other. In between these encounters, you'll be able to destroy one of the Sleeper's Hearts with the Uriziel sword.
You can find a detailed breakdown of the fight gauntlet below:
Harpies. Follow the corridor leading to a room filled with harpies. Defeat them all to progress.
Orc Warriors. Stand on the stone platform in the center of the area as several Orcs spawn amidst the sandstorm. You can defeat these enemies in one hit.
Mad Mud. Follow Mud along the mine shafts, fighting the Minecrawlers in your way, until you find the Mad Minecrawler Queen in a nest-like room. Take her down and follow Mad Mud to the end of the mine shaft, and then take him down to unlock the path forward.
Mad Diego. He'll start shooting at you right away, so make sure to move in between covers until you reach him. After bringing him below 50% HP, he will teleport to the opposite side of the area, and you'll have to reach him again as he shoots at you.
Mad Gorn. He has thunder attacks that can cover a wide area in front of him, so dodge roll around him, avoiding his heavy axe attacks and retaliating in between his combos.
Mad Milten. You won't be able to damage him while his shield is up. To deactivate it, stand near the unlit braziers and make him shoot his fire spells at them to light them up and shoot a fire bolt back at him. Repeat this three times, and you'll take him down.
Mad Lester. The only peculiarity of this fight is that he can use a spell to knock you away from him.
Mad Xardas. He'll stand atop one stone pillar and summon several skeleton enemies while he shoots magic bolts at you. Stand near the same pillar to avoid getting hit. When taking down the skeletons, he'll teleport to the center of the area and will be exposed, so run to him and hit him quickly.
After you destroy the four Sleeper Hearts, Mad Cor Kalom will turn into the Mad Demon, and you'll have to defeat him. He uses the same attacks as other Demon enemies, including a swipe melee attack, a fire-breathing attack, and some new ones:
Ground rift. A green rupture appears on the ground and follows you around, dealing damage if you stay on top of it.
Fire explosion. The Mad Demon will glow red and then release a fire burst around him. Block or run away to avoid getting hit.
Finally, after taking down the Mad Demon, you'll be able to enjoy the final cutscenes of the game as the credits roll. Congratulations on finishing Gothic 1 Remake!