This is what a human can become when using a behelit to sacrifice someone close to them.
An apostle is an individual who has sacrificed someone important to them to supernatural forces so as to be remade as a monstrous being of inhuman strength and power. A hideous egg-like artifact called a "Behelit" is needed for the sacrifice. When blood is spilled on a Behelit, the entities known as the Godhand are summoned, at which point the summoner offers their sacrifice to them and is granted an apostle form.
An apostle usually has two incarnations. Their standard form, which appears human for the most part but bears some inhuman deformity. Apostles typically have some superhuman asset in this form, like enhanced durability, strength, etc or in one instance flight. The second incarnation is a far more monstrous rarely resembling a human being, which grants them far greater power than their normal one.
Those who have encountered an Apostle in the past can often detect an unusual and unpleasant atmosphere that they give off.