dead star so maybe a star that was destroyed by that martial constellation??

No, I think it is the martial constellation himself

those are my thoughts as well, because i remember them mentioning that usually constellations protect their stars, but i believe that the martial constellation let his star die instead because he cared more about power or sth

No, it was explained that he didn't have a star which is why he built an artificial one. Then he kidnaps other constellations and steals their powers on his artificial star.

ahh okay, for some reason i thought he had a star ;;