A tracker is highly useful here. Go to the Sarnak Fortress in Lake of Ill Omen. Clear mobs in the fort until a sarnak revenant spawns. They are pretty rare and can roam around inside the fort. I cleared the fort for 3 hours and only saw 4 spawns. 2 of the 4 spawns were in the hallway that leads to the rear entrance of the fort (see map), the others were in different hallways.
You’ll need 2 items total from a sarnak revenant, and they only drop 1 at a time so you’ll need at minimum 2 spawns (if you are lucky you’ll get one then the other). The two items you’ll need are:
Foot of Candlestick
Stem of Candlestick
Note #1: While at the Sarnak Fort look out for the NPC a sarnak gem oracle. It normally spawns in the front courtyard of the fort. It drops Gem of Yet to Come, which is used in Quest #7.
Note #2: There is an alternate way to get Foot of Candlestick. Loot Stein (always drops) from halfling swashbuckler in Firoina Vie. He spawns on “mud hill”, but is quite rare. You’ll want to clear trash around the hill to spawn him. For Quest #7 you’ll need to spawn a different NPC at this location anyway. A tracker here is useful.
Give Stein to Grype in East Cabilis to receive Foot of Candlestick. Grype only spawns at night and despawns at approx. 7am game time. Clerk Doval spawns in his place during the day.
Go back to Master Kyvix in West Cabilis. Give him Foot of Candlestick, Stem of Candlestick, and Occultist Skullcap to receive Revenant Skullcap. I did this turn-in at Kindly faction, but you should be able to do it at either Amiable or high Amiable (unconfirmed which).
You can now move onto >>Quest #7<<