Well they just seem like death entities honestly. Someone of the Lich Kings power over death should be able to make them since it’s obviously not exclusive to Odyn and Helya.
It might really be like banshees, being a special type of undead created from a certain race. Vrykul ladies might be able to become it by an infusion of thicc death energy.
this is all i got from your post
Have Legion raids been divied out to Factions yet?
If Helya is canonically killed by the Alliance - I can see Helya being more willing to help Sylvanas when she returns. She may be able to give her more gifts and knowledge. They do have some sort of deal.
If Helya is canonically killed by the Horde - that would set Helya up as another enemy against Sylvanas. If Sylvanas double crossed Helya and had the Horde eliminate her - well, I can see Sylvanas using Xalatath to contain Helya later.
not yet, gotta wait till chronicles 4 or 5 to figure out who did what. though its likely neither of the factions get credit and they simply give credit to the third party order halls
So the boss would basically be Elisandre with one more phase?