Dont know if this drops from anything else but i got the Control Gem quest item from a teal Trex. I believe the Trex was called Slythed of the Deep.
I found him roaming around the middle north of the map (Twisted Caverns)
Once i got the drop, activate the quest and turn in. Bloop achievement.
The map is Twisted Caverns
Slythed of the Deep around map coordinates ( 793, 3885 ) close to a treasure chest.
(His pathing is circling the rock at (828,3850)
Torkam Stonesorrow at map coordinates (387, 3816)
Thanks to Preeeemo for the confirm on whether it drops from anywhere else.
Im guessing there is probably 1 more named that drops it, as there seems to be 3 named characters to a zone