as redittor says (source https://www.reddit.com/r/wow/comments/kwb35e/if_you_use_a_ravenous_anima_cell_on_a_lost/glia5mq/?context=3) :
He showed up near the end of my run. Best to clear everything and avoid attacking him. You have to hit him with an AOE as you cannot target him. Don't try to kill him. He walks towards the end of the level, dropping white orbs along the way that you can pick up by walking over them. They give phantasma. He stopped half way and I had to hit him again. Right when he got the the portal at the end of the level, he automatically dropped an anima orb, which didn't have anything special about it. I quickly killed him before he ran away after that and I didn't get anything else.
It is. I just saw one today there :)