Monster to Tomo ni Level Up ~Shadow Sovereign~

Fifty years ago, a massive tower filled with monsters appeared on Earth, trapping humanity inside. The only way to escape is to awaken as a hunter and conquer the 100th floor. Yushi, born on the first floor, works harder than anyone yet remains unawakened and powerless. Everything changes when he merges with the Shadow, a forbidden monster fleeing from the upper levels, granting him unprecedented power. The cost is a secret quest ordering him to fight alongside monsters rather than destroy them. Target of nobility seeking his new power, Yushi must navigate conflicting missions and deadly pursuers as he uncovers the truth behind the tower. Original Webtoon