Zaven The Alchemist is a merchant and alchemist.
The location is close to the entrance to the Oasis. It is the first path that opens south of you past Cain's Lost Satchel. Soon after that there will be a shallow opening full of water that leads you south. If he is here,he will be surrounded by enemies which you have to kill to save him. If you are able to kill these adds in time, he will thank you and open up a door leading you into a waterway.
There is a second place where he can spawn. Keep following the south wall from the first place he spawns, all the way to the east. It is much easier to keep going to the first spawn location near the path to the Dahlgur Oasis. Watch the video below.
When you are in the waterway, the chest looks just like any other ordinary chest,it has the label "Mysterious Chest". You may have to clear the first and second levels of the waterway to find it. There is also a chance that it may not spawn at all. This is the issue for some people after saving Zaven, as looting the Mysterious Chest yields the Liquid Rainbow, one of the items necessary to forge the Staff of Herding, the item that you need to access the secret area of Whimsyshire.
If he is not at either point, leave your game and then resume and try again. It may take several tries to find him.
Talking to the Zaven is one of the criteria for completing the Market Research achievement, along with:
- Rodger the Alchemist
- Vendel the Armorsmith
- Adenah the Curio Vendor
- Kyr the Weaponsmith
- Wandering Tinker
- Halmin the Alchemist