After the elf died horribly, Arthur went close to the body and absorbed its essence. That was what he needed to do to complete his technique, allowing him to return to the living world fully.
His aura of a rank-three mage vanished as the elf turned into a mummy, and its essence went directly toward the rune carved on Arthur's skin.
It was just a temporary level-up, one Arthur lost after completing the technique, but he was not feeling down because of it, as it was a great spell that saved his life.
"There are many corpses in here with blood. I should use them for training," thought Arthur, sitting cross-legged amidst an ocean of corpses.
Although the level-up was temporary, the environment was still excellent for his training, and as the fight against the elves had not taken long, why not use some of the free time to practice?