When you have finished the 2 manticores and the shade that follows you will see darkness form the shade shoot out to the nearby monolith and destroy it. This reveals an opening leading to a hallway which end's in the Breach Chamber.
The floor of this room is inscribed with circles, with triangles between the circles. Once the fight starts, every so many seconds, white light will come out of the holes in the room except for one part. The light is followed by spears, which deal a lot of damage (1200-1500 damage). Tank the boss at the border of two circles and move whenever the white light is coming out of the holes to a circle that is not lit. Once the spears came out, you can move into them without receiving any damage.
Konstantin has a debuff that makes him take 20% more damage from attackers within 3.5 meters.