Overeater

Overeater (喰らい尽くすもの) is a demonblade currently in the possession of William. It was originally owned by the High King.

The Boy in the City of the Dead
The demonblade, Overeater, was given to William as proof of his independence by Blood after their final duel against each other. It was one of the strongest demonblades in his possession and was originally owned by the High King.

The weapon can cut down anybody, including echoes, as long as the blade reaches them, the blade is inscribed with many words too difficult to figure out, however, it's effects are able to be described in one sentence...

"As you cut living being with this sword, it recovers your life force. The more you cut, the more you recover."

William was warned by Blood, not to rely on this weapon, as he would grow too dependent on the weapon, he takes his words to heart and tries his best to only use the weapon when required.

In the first instance, William has used the demonblade against the splinter of an echo, belonging to the god of undeath, Stagnate. He was able to cut through his body multiple times using the weapon.

The Archer of Beast Woods
In the second instance the sword was used against an ambush of wild animals and serpents lead by a Chimera. He lost control of his emotions after witnessing his closet friend, Meneldor, take a heavy blow against the Chimera, filled with anger and hatred he slaughtered the animals and serpents without any real strategy or grace, making him feel like a failure in the eyes of what his foster family has taught him.

The Lord of the Rust Mountains
The third and final instance was when he took a direct and final attack, consumed by miasma and scorching heat from the vile dragon, Valacirca. His body was not able to withstand the intense heat and poison and started to melt, before losing consciousness and ready to accept his fate, he heard the voice of Mary, determined to live, he pulled out Overeater and slashed repetitively at the dragon, regenerating and melting constantly until he lost consciousness.

Ability
Recover Life Force: The Demonblade is able to absorb any matter of life force allowing the user to drain it's enemy in order to heal themselves.

Blade: The Demonblade can be used to cut everything including Echoes and the sharpness itself cannot be matched by any kind of blades making it the most powerful weapon.