>>714000879
Because he's at the lowest point he's been since Black Swordsman after the Elven Island disappeared and Griffith took Casca. He's as suicidal as back then except now he has a party of people to care for and Casca's in active danger vs having been left with Godot, but still he wants to die. The tomb's not gonna be a literal tangible power-up, it's just gonna fix his head.