Our friends at RiftBoss created a guide for Warmaster Galenir in the new River of Souls 20-man raid instance. I have yet to raid in the new instance but the fight looks pretty straight forward. If you are familiar with ICC, then this fight will remind you of Festergut with the stacking debuff that will require you to swap tanks at around 8 stacks.
Enjoy the video below and give them some support!
You should subscribe to their youtube channel if you really are interested in viewing more of their guides.
Good luck on killing Warmaster Galenir and have fun in the River of Souls!