Crystallized Horror is a Sha-Touched gem, fitting only in a Sha-Touched socket.
This item is sold by the following vendors for 10000:
In order to purchase it the player needs to have completed  Breath of the Black Prince before patch 6.0.2.
See Legendary Legion equipment
[Ashjra'kamas, Shroud of Resolve]
[Rae'shalare, Death's Whisper], also see Legendary Shadowlands equipment