Not only is the encounter itself extremely bugged (won’t appear out of the sprawling animation after inserting all gem stones) the phasing surrounding it is, too.
A friend of mine needs to kill it for the rank 2 recipe, and has been trying to for the past week or so since when he completed the quest you get at the beginning of the Nazjatar storyline he didn’t have the profession yet.
Now, just a few moments ago, I stumbled upon one of the very few existing phases were Avarius hadn’t been bugged yet (i.e. : the original Brinestone deposit which you need to “mine” with the Brinestone Pickaxe was still there), so I invited my friend into a group to get him into my phase while waiting for him to arrive.
However, in the meantime some other players arrived and started mining the gemstones to summon Avarius. Eventually they succeeded in summoning him (a feet in itself, managing to get him to appear at all) and started killing the rare mob. In the meantime, my friend (who is at the same repuation and storyline level as I am in Nazjatar) arrived, outside of my phase (even though I was group leader) so was unable to interact in any way with Avarius. He got annoyed and left the group, which led to ME getting phased out of the fight with Avarius, into another bugged phase with the bugged summoning already completed and without my friend. Then my friend reinvited me, and I got phased into his phase, with another bugged Avarius encounter.
TL:DR to summon it up: a) The encounter itself is incredibly bugged b) the phasing surround it is bugged. I invited a friend into a group that was in a phase with Avarius currently active, however my friend was denied phasing into the same phase. He left the group (group disbanded) which led to me getting phased into another bugged phase too. He invited me into his group, and I instantly got phased into his (also bugged) phase.
These bugs have existed since the patch launched, please fix it.