Initially he has :Absorption on him, which make him nearly impossible to defeat.
To remove this effects, the three braziers must be lit. Two are located to the left and right of the entrance, and the last is behind Remmenaeg. The challenge requires only lighting them twice (DPS race).
Lighting the right brazier will spawn a swarm of bats, and the left brazier will summon Fell Spirits. Lighting the rear will spawn a spirit that should be CCed.
First, let the tank build threat on boss, pull him to the brazier that spawns bats, away from the add that needs to be mezzed.
Second, light all the braziers and return to DPS boss until braziers go out, AoE to remove interrupting bats very useful.
Third, clean up any adds, recharge, wait for DPS CDs to expire.
Rinse and repeat. (If didn't get boss down more than 50% first time, reset fight.)
Remmenaeg has an aura of 6 Dread, which makes the fight a lot more difficult. However, characters standing close to a brazier will receive +2 hope, reducing the dread. Use Edhelharn Tokens to reduce dread even more.
Drops
Incomparable Drops
Quotes
"None shall escape the clutches of the champion of Angmar."
"There is nothing you can do to stop her."
"You may have stopped my brothers, but your time comes to end now."