Antorus, The Burning Throne - Mythic: Felhounds of Sargeras - Guide in Description

This fight has three changes on mythic: At the start of the encounter half the raid gets flametouched and the other shadowtouched. You want to stand near the Hound that matches your colour, as you take 25% less damage from that element.

When you're shadowtouched and take fire damage, you death grip the nearest player to you. Also Molten Touch will hit you extremely hard and you'll want big defensives or immunities if targeted for it.

When you're flametouched and take shadow damage, you drop fire beneath your feet that starts doing damage 4 seconds later.

At the pull the tanks need to figure out who has which debuff and taunt the matching dog so you're not constantly death gripping or dropping fire and take 25% less damage from breaths. Due to the dropping fires you'll want flametouched players to move in a circle around the room, with the flame Hound moving away from the shadow Hound while the shadow Hound stands still.

A little after 2 minutes in the debuffs will reverse, the tanks should switch bosses and everyone makes sure they follow the matching hound to their colour.

When Siphon Corruption happens and you're flametouched, you'll want to stack up and move around the boss in a circle every time a new flame patch is dropped. It's a good idea to mark a player to follow for the flame side and have everyone always follow that player during siphons.

The other change on mythic is the Consuming Sphere will now have a target area marked by a giant purple swirl and explode on impact, instantly killing anyone in the swirl and doing a small amount of shadow damage to the raid. This will spawn flames under flametouched players, try not to drop them in the path of where you need to go for your boss kite.

The fight revolves around learning the dangerous combos.
~1:40 Siphon+Molten Touch: The hardest part of the fight. You'll have to move a bit faster than normal for this when the patches appear under you, and no one on shadow side can get hit or they'll pull another player into instant death. It can also be hard to heal the touched players, healthstone+defensive! If the touched players get siphon they're dead unless they get back in the group very quickly.
~3:00 Desolate Gaze+Siphon Corruption: Siphon will one shot you on mythic if you're alone, so you can't move far for these gazes and need to stack immediately after they fall off.
~4:20 Enflamed Corruption into Siphon Corruption: Again, you can't be too far here. Getting caught out alone with siphon is death.
~5:30 Enflamed Corruption+Siphon Corruption: This is the soft enrage, you will probably lose most of the raid to this. Immunities can get you through it but many players will get siphon+enflamed and will almost certainly die. The bosses should be dead before this.

Also, the bosses have separate HP bars on mythic and need to die within 15 seconds of each other. The flame hound tends to die slower due to its mechanics, so while you're swapping at ~2 minutes and have a bit of downtime everyone should dps the flame Hound to even their HP out.