This fight has two phases, each with completely different attacks. Once he dies in first phase, he resurrects shortly after at full health in second phase. His vulnerability completely resets at each of his resurrections.
In the first phase he welds a sword, his basic attack is a vertical slash in front of him that damages and interrupts spells. [Hard+: leaves extra soul fire in the area]
In the second phase he welds a staff, his basic attack launches a ring of pestilence towards his target.
He passively regenerates 1% energy per second, and gains an extra 0.5% energy per second after each of his revivals.
[Hard+: He will respawn again in a 3rd phase, starting at only 50% of his max health this time, 6600 health. In this phase he will swap weapons very often and will have access to all of his moves at any time. He also now regenerates 4% of his energy per second, finish him quickly]
4 gravelings periodically get summoned at a random location, these deal 150 damage with their melee basic attack.
Summons a massive amount of marks on the ground, a short time after they explode, dealing 750 damage in a small area each. A deadly move at close range where marks are most clustered. A single mark will also always spawn directly on top of every player. Counterable
Worth a most of times as it is very hard to fully or even partially dodge that move. If you stay in movement as he casts it you have just the right amount of time to walk away from the mark on top of yourself.
Summon gravelings that start attacking players. If you don't kill them fast enough Narthuul devours them, restoring a portion of his energy. They can get devoured very quickly, this is indicated by a timer below their lifebar. Counterable
Heals back 30% of his max health and regenerates an extra 20% of his max energy over 8 seconds. Counterable
Narthuul becomes invulnerable and teleports behind a player to strike him with a much bigger vertical sword slash that deals 2000 damage, interrupts players hit, and briefly slow them. [Hard+: leaves soul fire in the area]
Narthuul quickly throws his weapon in front of him, dealing 1100 damage per second and slowing players in its path.
Summons a pestilence zone in the middle of a soul circle around Narthuul. The soul circle deals 250 damage per second and heavily slows players that attempt to walk through it. They both last for 8 seconds and move slowly in a random direction. Counterable
Chains the boss to a fixed random point around him. If the boss remains chained he will start regenerating 0.5% health and 2% extra energy per second. To break the chain a player has to go to where it emerges. Doing so will however get that player chained for 5 seconds, slowing him down, pulling him back if he runs away and dealing 600 damage over that duration.
Players pushed by his ultimate into a chain point will not be chained.
Traces 6 trails of soul fire from the boss. Soul fire deals 400 damage per second and lasts for 5 seconds. Narthuul won't use this spell two times in a row. Counterable
Launches 5 pestilence zones in an angle in front of him. These zones last a very long time.
Marks the ground several times at random locations, the marks explode after a short delay, releasing 5 piercing soul missiles each. Soul missiles deal 1000 damage per second and apply Decay to players hit. Counterable
Narthuul charges his weapon for 2 seconds, then hits a large area in front of him with a powerful slam, dealing 1250 damage and applying Cripple to players hit, slowing them for 10 seconds.
Summons a necromancer that channels a short time before crashing into a player, restoring himself to full life, knockbacking, dealing 750 damage and applying Decay to the player hit. He repeats this sequence as long as you don't kill him. He may change target.
Summons 3 necromancers around a player that start channeling, when they finish they all crash at that point, fusing into a netherbeast that pulses for a few seconds, dealing 1500 damage and knockbacking players in a large area every second. The netherbeast slowly moves towards a player. Counterable
The boss teleports at a favorable spot and does a very wide horizontal slash with his sword that deals 2750 damage and heavily knockbacks players hit. You most likely have to this move but you can also dodge it by getting out of his reach, or by being on the left back side of the boss (from where he starts swinging his sword) as his slash will only travel an angle of 270°.
Afterwards he raises his staff and instantly zaps all players, applying Decay and a stack of Doom to them.
Decay is a debuff that deals 800 damage over 8 seconds.
Doom is a debuff that reduces maximum life by 250 per stack, doesn't disappear even beyond death and stacks up to 1 [Normal+: 3 [Hard+: 4]] times. If the target is already fully stacked, it deals 250 damage instead.