Go to the rancor cave that has been reported by the Nightsisters/Singing Mountain Clan.
Travel to the waypoint provided by Fath H'ray/Izaryx
After you've arrived to the waypoint, the quest will update
Collect Blood Samples from Mutant Rancor
Comparing some mutant rancor blood samples with the mutant rancor teeth you already gathered should help to determine the cause of the increase in mutation.
You are tasked to collect three blood samples from mutant rancors. Make your way down the cave and slay mutant rancors until you have enough blood samples. They will NOT always drop them
The cave is full of normal rancors and mutant rancors
Mutant rancors are CL 89 elites with special buffs:
Mutated Skin: Reduce damage taken by 5%/10%/15% - can be up to 3 stacks
Unnatural Strength: Damage output modified by 15%/30% - can be up to 2 stacks
Razor Claws: Increase chance to bypass part of your opponents Armor by 15%/30% and Possible amount of Armor bypassed on a strikethrough attack is modified by 20%/40% - can be up to 2 stacks
Because of these buffs, they are a tough fight
After getting all of the mutant rancor blood samples, the quest updates
Return to Fath H'ray/Izaryx
Take the blood samples you've gathered back to Fath H'ray/Izaryx the Mistress of the Rancor.
Make your way back to Fath H'ray/Izaryx at the Nightsister Stronghold/Singing Mountain.
When you arrive, speak with Fath H'ray/Izaryx.
Fath H'ray/Izaryx: You have the blood samples?
You: I do.
This completes the quest. Continue your conversation to move onto the next quest.
Reward: 4 Rancor Teeth and 4 Random Valuables
Fath H'ray/Izaryx: Excellent. Let me compare these with the teeth you provided earlier and see what can be seen.
You: Can you pinpoint their origin? Fath H'ray/Izaryx: Yes, with the aid of Spell Weaver Leandra/Arch Witch Azzenaj, I was able to. Go there and find the source of the mutations.