The Beginning After the End

From Yen Press:
Deep inside the dungeon, Arthur and his allies come face-to-face with a mana beast of legend, the S-Class Elderwood Guardian! With death imminent and his party members quickly descending into madness, Art knows he’ll have to be the one to guide them to safety. But no sooner does he go to help than Lucas turns against them! Wounded by a so-called ally and being attacked on all fronts, Art makes a choice. It’s time to bring out the next stage of his beast will even if his body can’t handle it.