The Witch of the Wastes is a main story quest chain in Diablo 4 that takes place in the Cinder Wastes, which directly follows the Swamp Hospitality quest. Overall, this is a fairly easy series of tasks that should not take you too long to complete, though players have reported bugs around placing Purified Quicksilver on the table at the end of the quest.
If you are trying to complete the Witch of the Wastes quest, or simply trying to place the Purified Quicksilver to complete the chain, here is what you need to do.
Where to find Valtha in Diablo 4
The first major task in the Witch of the Wastes quest is the travel from the Ruined Tower to find Valtha’s Hovel in the Cinder Wastes, which should be marked on your map. Once inside the Hovel, walk over to the witch’s den and click on the Manifesto laying near the cauldron.
By clicking this, you will spawn Valtha and start a the boss fight after some brief dialog. This fight shouldn’t be too challenging, and I managed to clear it without dying. Just make sure to avoid Valtha’s AoE attacks and clear out additional enemies that spawn throughout the fight.
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How to place Purified Quicksilver on the table in Diablo 4
Once you have defeated Valtha and acquired the Purified Quicksilver, you will need to venture back to the Ruined Tower. You should note that there is a bug in this quest that will not allow you to place the Purified Quicksilver on the table once you reach Donan at the Ruined Tower.
Though the exact cause of this bug is unknown, players have reported being able to skip this issue by venturing back to the Ruined Tower on foot instead of using the nearby Wayport. While I did not have any issue after using the Wayport, it still seems to be a possible cause of the problem for others.
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After you enter the Ruined Tower, there should be a prompt that tells you to place the place Purified Quicksilver simply by clicking the left mouse button. Do this, and the quest will finish after more dialog.