One of the pets necessary for the safari achievement, located in the Vaults of Ula’tek special event zone. Yesterday I decided to make a suicide circuit around the area; saw a bunch of Cursed Spawn around the venom lake before getting to the Amani outpost on the left side. Rebuffed then continued my suicide run up the stairs and it was on the second tier that I found a Caustic Writhling going down towards the outpost. Managed to bag it just before the cursed skeletons killed me.
So, getting this pet will be a pain in the butt because of the army of baddies, but at least you can retreat to the outpost between suicide runs.