The Soul Tyrant is the Dream Version of the Soul Master.
In order to fight him you have to return to his corpse in the Boss room in the Soul Sanctum above the City of Tears.
He is a lot faster in this version of the fight and you dont have many chances to hit him.
Try to play it save during the first phase and get hits in when you can, then burst him down in the second phase with your Howling Wraiths / Abyss Shrieks when he shoots his projectiles.
Level up your health/nail as much as possible, you can use Fragile Strength/Life charms for this as it is a dream fight, which means they wont break.