Absolute wall of text inc. I played and adored Demonology in Legion/BFA, but if anything huge has changed in Shadowlands I may be wrong.
Demo is one of the most hard-cast centric specs in the game, playing around Soul Shard generators (Shadow Bolt, Demonbolt). They have two spell schools, Shadow and Shadowflame.
Their sustained damage rotation essentially just consists of generating Shards and then dumping them with Hand of Gulâdan (Shadowflame, up to 3 Shards, Imp per Shard), and Call Dreadstalkers (20 second cooldown, 2 Shards, Shadow).
Their burst involves summoning as many demons as possible, ideally a set of Dreadstalkers and 6 Imps, alongside possibly a second Felguard via Grimoire: Felguard, before casting Summon Demonic Tyrant, which buffs the damage of all existing demons, extends their duration, as well as does some damage of his own.
Summon Demonic Tyrant itself is a cast. When they attempt to burst, make sure to kick or otherwise delay Summon Demonic Tyrant as this will not only stop the cast, but also often cause them to miss the timing as their other demons begin expiring.
If they arenât bursting, aim to kick Hand of Gulâdan, as this will lock out their entire kit. If you kick Shadow Bolt, they will still have access to Hand of Gulâdan and Implosion.
With regards to fighting them, they are vulnerable to all of the same mechanics that all pet classes are. Verticality will significantly hamper the damage of the pets, and line of sight is particularly effective against their Imps and Demonic Tyrant.
Killing their main Felguard will significantly reduce their pressure, as they lose access to its sustained damage and Mortal Strike. This will also disable Soul Link and make them super squishy. Fel Domination can be used on a 3 minute cooldown to cheat out a new pet, once thatâs down itâs pretty much game over.
You probably wonât have much luck simply PvEâing them midfield as BM. Despite how horrendously clunky they are, the amount of damage they can output if allowed is probably one of the most slept on aspects of arena.
Abuse line of sight to deny them hard casting on you, as they have no real mobility or slow. Demonology has a large array of CC, but it really wants to use it to secure kills, not to get damage out in the first place.