Overeater was given to William as proof of his independence by Blood after thier final duel together. It was noted to be one of the strongest demonblades in his possession which 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 independent on the weapon, he takes the words to heart and tries his best to only use the weapon when required.
The first instant he used the weapon was against the splinter of an echo of the god of undeath, where the blade was able to cut through his body multiple times.
The second instant was against a surprise attack from a group of wild animals and serpents.
The final instant was when he took a direct attack from the final attack, consumed with miasma and scorching heat from the vile dragon, Valacirca. His body not being able to withstand the intense heat and poison starting melting, hearing the voice of Mary, he pulled out Overeater and starting slashing at the dragon, regenerating and melting constantly until he lost consciousness.