Soul Master

Soul Master was the leader of the Soul Sanctum. In an attempt to stave off the Infection, he hoarded soul and tried to attain a pure focus. It is revealed that the Soul Master was approached in his dreams by presumably the Radiance, who promised he would live forever if he took the soul of bugs. This was possibly the reason why the Pale King opposed the Soul Master’s plans.

When the Soul Master is defeated, he leaves the spell Desolate Dive behind.

Phase 1
Soul Master will teleport at least one time in between each of his attacks:

After taking a certain amount of hits, he will temporarily "deflate". Attacking him or waiting long enough will cause him to resume attacking.

His Soul Shot can be dodged by jumping or dashing either left or right if the Soul Master is far, but it wont hit the Knight if they stand directly next to him.

When he summons the four orbs that orbit around him (Clock attack), approach him when he is about halfway across the arena, jump over the bottom orb when it's closest to the ground.

The easiest way to damage him is to keep attacking him from below when he is conjuring the homing orbs.

He leaves many open opportunities to heal, notably when he "deflates" after taking enough damage. Staying in the center after he finishes his Clock attack also is a good opportunity to heal multiple times.

Phase 2
After taking enough damage, the Soul Master will appear to have died; with a long drawn out death sequence ending with him leaving behind his signature spell. However, just before before the Knight can absorb the spell, he reappears, breaks the ground and the battle enters its second phase.

In this phase, the Soul Master only has two attacks, which he will alternate using:

This phase is overall significantly easier than the first phase due to the predictable nature of the Soul Master's attacks, the lack of shockwaves on his Slams, and his openness to attacks during his projectile attack. Keep dashing and jumping in either direction when he does the Slams. Once he starts conjuring orbs, he is open for 2-3 or more hits each time, but make sure to keep moving to dodge the homing projectiles.

Learn to avoid the projectiles and attacks, as this phase will give the player much less time to stop and heal.

Location
=Soul Tyrant=

The Soul Tyrant is the Dream Boss variant of Soul Master accessible upon returning to Soul Master's Corpse and striking it with the Dream Nail.

Phase 1
Soul Tyrant uses all the same attacks as Soul Master, but at a much faster pace with minor adjustments to some attacks:
 * Clock: Soul Tyrant now summons six (two rows of three) orbs that rotate around him.
 * Slam: The shockwaves produced from the slam are taller, larger and travel at a faster pace.

Phase 2
Soul Tyrant teleports much more rapidly than Soul Master which is one of the main difficulty changes to the fight. It can be very difficult at times to score hits on him and fill up SOUL, so charms like Soul Catcher and particularly Soul Eater are quite useful.
 * Altered Slam: The impact radius of each slam is much larger.

Despite his Clock attack being modified, the time right when the orbs start to travel away is still the best time to heal during the fight- right in the middle of the arena.

For Phase 2 there is virtually no more time to heal as his orbs come at a non stop pace one after the other. During this phase Soul tyrant's health would also be considerably low so using high damage spells like Abyss Shriek or Desolate Dive on him while he summons orbs is a good way to kill him quickly before too much damage is taken.

Trivia

 * Soul Master is the only boss to give the Knight a Spell after being defeated, that spell being Desolate Dive.
 * In the game files, he is referred to as "Mage Lord".

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