Death Magic seems to include Nature not just with the Drust but the Night Fae of Ardenweald too.
Death Magic doesn’t include Holy as far as we know(which might change depending on revelations concerning Xal’atath and Beledar).
As for what I suspect concerning Xal’atath and Beledar: When Alleria said “enough of your illusions Xal’atath” the Beledar immediately turned to it’s Holy phase suggesting she is the Beledar.
Furthermore the Elementals that Xal’atath utilizes(which use a purple version of the Death Elemental Model) use Death Bolts, Death Nova and Death Rain as attacks.
Xal’atath also had Domination Runes(the invention of the Primus) on her Dagger form.
If Xal’atath is infact an Eternal One imprisoned in the Beledar by the Primus(most likely the Winter Queen as the current one is the only one who doesn’t treat the other Eternal Ones as family) who had her eye captured and contained in a Dagger with Domination and is of Death then she utilizes Holy Death Magic visually indistinguishable yet fundamentally different from the Arathi’s Holy Light Magic they brought over from their Empire.
For Death it utilizes every School(Holy, Nature, Arcane, Shadow, Void, Necromancy and Cosmic) but Fel!
Hot Summer Day is Holy, Cold Winter Night is Void, Sin/Revendreth & Domination & Pain/Mawsworn are Shadow, Necrotic Energies is Necromancy, Twilight(what Ardenweald’s Magic is called by Spell Names) is Nature, Virtue/Bastion & Hyperlight(Broker Death Magic according to Spell names) is Arcane(according to Damage) and Automa Magic is Cosmic(according to Damage).
Holy Death Magic is the closest visually to Cosmic/True Death Magic just as Holy Light is closest to Cosmic/True Light.
The fact that all 6 Cosmic Forces have 6 Schools while leaving out a 7th(Fel in the case of Death and Order) suggests that the 7th School is the anomaly for each of them. The solution awaiting it’s opportunity! The 7th Covets what the other 6 hold fast! The fulcrum wanes. All will be undone!
Zovaal leaving a crack in Zereth Mortis will bring about Fel Death Magic! Fortunately Cosmic has a representative in the form of Pelagos so Fel will simply counter the Brokers(disrupting their business practices no doubt) just as the current Winter Queen’s Nature counters the Primus’s Necromancy, the Archon’s Virtue counters the Court of Harvesters’ Sin and Hot Summer Day counters Cold Winter Night.