Skin Concept: Dark Star Guardian Kayn

Finally, a champ to fill both fantasies at once. Dark Star Guardian Kayn. Normal: a half-corrupted human star guardian. Losing its light from the first star and getting consumed by a black hole. A fracture is visible on its star jewel on his chest, and a black hole is visible through it. Wields a scythe with the same jewel design. Shadow Assassin: dominates the foul Darkin (Dark Star)'s corruption and becomes a star guardian. His star is mended and now glimmers with light. Darkin: His soul is completely consumed by the will of the dark star. There is nothing but void left.

Extra note: I guess we can assume that the green monsters from invasion are Dark Star's minions/or completely different entities ("a bigger threat was still remaining..."