A Greedy God - Velen
Location: South of Wastrel Manor
Quest Giver: Dagmar
Other Requirements: Obtained The Eye of Nehaleni from Keira Metz's quest chain in Velen
You can find this quest by traveling south of the Wastrel Manor Signpost, while doing the A Favor for a Friend side quest from Keira Metz you'll pass right by the NPCs that give this one. After accepting this quest the first thing you will need to do is use the altar next to the quest giver to have a word with the Allgod.
Once the conversation is over use your Witcher Senses on the bottles of vinegar next to you to acquire a scent. Follow that a few years away from where you found it to discover an illusion. Use the Eye of Nehaleni to reveal a staircase that you'll follow down to meet the Allgod.
Upon discovering the Allgod you have a few options for dealing with him. None of these options have any effect on the greater story outside of this quest. For the best reward you'll want to bargain with the Allgod and have him accept their current offerings.
Tip: Sparing Allgod's live will give you credit towards helping monsters in the Contract: Skellige's Most Wanted Quest.
Mentioning the cellar to the peasants after speaking with Allgod will get you more EXP and Crowns as well.