The Giant

The Giant is the fifth boss in Dead Cells. He was added in the Rise of the Giant update. He lays dormant in the The Prisoners' Quarters till the player beats The Hand of the King. Once The Hand of the King is beaten, The Giant will awaken from the starting cell and bust down the door found after the starting items. Once the player follows him in, a small cut scene will play. In that room can be found a key that permanently unlocks The Cavern which can be reached from the Graveyard.

His boss fight can be found in The Guardian's Haven which can be reached from the Cavern.

Lore
The Giant used to be part of the Royal Guard, as indicated by the huge painting in the High Peak Castle representing him under the King's banners. His duty was to guard the gates of the Castle. However, during the events following the Malaise epidemic, the Giant started voicing his concerns and warning the King that his methods were leading the island to its ruin. Following this feud, the King had the Giant killed with a spear and his corpse dumped in the Prisoners' Quarters. While the King gave instructions to make sure the Giant would never rise again, beating the Hand of the King wakes him from his slumber.

When the Beheaded reaches the Giant for the first time, he is revealed as the King of the island, who has lost his body after suffering from the Malaise (and probably due to the Alchemist's experiments). The Giant has a poor opinion of the King/Beheaded, but still respects his title and seems to keep a great respect for royalty and the kingdom. Indeed, the Giant is the only character in Dead Cells to call the Beheaded "My King..." (except for a small cutscene with the Hand of the King). Upon being defeated, the Giant once again expresses his frustration and disappointment with the King's actions, and eventualy disappears in the lava after calling him "an incorrigible ass" and giving him the middle finger.

Upon reaching the Giant with 4 BSC active, he will bear a crystal identical to the one on the King's chest on his forehead. After defeating him, a different dialogue than usual will trigger where the Giant admits his defeat was fair, unlike in the old days (referring to when the King had his soldiers kill him with a spear). He will then gift the 5th Boss Stem Cell to the player, noting that it might be of more use to him, before his head explodes. This cutscene cannot be seen again once the Boss Stem Cell has been absorbed.