In Chapter 5 of Gothic Remake, you'll find the Uriziel sword inside the Sleeper's Temple. After talking to Xardas, he will instruct you to power it up using the Water Mages' ore pile. You can find more information on how to obtain the Uriziel Sword in our Gothic 1 Remake Sleeper Temple Walkthrough.
After obtaining the Uriziel Sword and the five crystals from the Orc Shamans, speak to Xardas, who will ask you to channel the magic power from the Water Mages' ore pile into the sword. He will give you the Power Exchange Scroll, necessary to conduct the ritual, and direct you to Milten.
Powering up the Uriziel sword
You can find Milten at the Water Mages' hideout in the New Camp, inside the room where the teleportation crystal leads you to.
Speak to him, and he'll direct you to Gorn, who can help you get past the guards. Gorn is usually standing near the back end of the New Camp cave. He will allow you into the ore chamber, asking you not to kill any guards in the way.
Go back to Milten and follow him into the ore chamber. You'll have to knock out a few guards on the way down. Once you reach the ore chamber, Saturas will show up next to you. Your choices on this dialogue could trigger a fight with him - Make sure you select "Your plan won't work, and you know it" to avoid confrontation.
After the cutscene, talk to Milten and teleport out of the New Camp, as the guards and the Mages will be hostile to you. Go back to Xardas to continue the story.