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You just need the potion 'Cure Dark End's Poison'. Are you sure you have the right potion?
I have that potion but the quest is not clearing. Anything else I need?
All you need is the potion. Are you sure you cooked it?
Eustacius the Alchemist in Monument of Obereg tells you, "If you have a sufficiently high alchemy skill, you can cook these potions on the brazier outside and then let them smell it."
Tap the sphere outside his house with one cure poison potion in your inventory. If your Alchemy level is at least 90, you'll get the Cure Darkend's Poison potion.
If you have more than one Cure Poison potion in your inventory, drop the excess, tap the sphere and then pick up what you dropped.