The boss has multiple phases with different attack styles. During all of his phases, contact with his attacks or any of his minions' attacks will inflict Shadowflare Flames.
Fighting the boss without Asthraltite armor will inflict the Shadowflare Vortex debuff for the duration of the fight, halving your damage output, nullifying your regen, disabling lifesteal and making Shadowflare Flames deal more damage.
Nihilus cannot move in this phase.
At the start of the battle, spawns a ring of orbiting Abyssal Embers which fire abyssal seeds at players. The embers are invincible for the first few seconds of their lifetime, and Nihilus is invulnerable while any embers are active. He resummons the embers at certain HP intervals (87.5%, 75%, 63.5%, 50%, 25%), and the amount of embers summoned increases as Nihilus loses health.
While any embers are alive, Nihilus will not attempt to directly attack the player.
Fires abyssal seeds at the player.
Fires abyssal fireballs which home after the player after colliding with the ground.
Fires seeds upwards which fall down and shoot at the player once colliding with the ground.
Summons groups of Abyssal Lanterns that shoot abyssal seeds at the player. When destroyed, they release falling seeds that fire themselves at the player when they collide with the ground.
After falling below 66% health, he will begin summoning damaging vine pillars, indicated by a thin pink beam acting as a telegraph. When a second vine pillar is summoned, the first one will vanish in short order.
Begins flying after the player in an attempt to ram them.
Attack speed for all attacks increases.
Becomes invulnerable until the Aura of Suppression is destroyed with a Relic weapon as instructed by Erazor. The Aura of Suppression has 200,000 / 500,000 HP and takes no damage from any non-Relic attacks.
Attempts to fly into the player, dealing massive damage on contact.
Summons several flame projectiles in plus-shaped patterns that fly outwards.
Summons flame projectiles that spiral outwards from their point of firing.
Makes his claws orbit himself and tries to ram the player.
Once the Aura of Suppression has been destroyed, uses all attacks from previous phases, but with increased speed and damage.
When below 50% HP: Summons rings of stronger Abyssal Embers around each of his claws, which fire Abyssal Seeds at the player.
Once Nihilus is killed for the first time in True Mode, he gains stat boosts during any future rematches.