This boss is known as Conquest or Pestilence in the Holy Bible, instead of Plague, he was supposed to be the first Horseman.
His weapon, Contagion Crossbow, just like it is described in the Bible as Conquest.
Ironically, he wears a Plague mask, a unique bird-like beak mask used by doctors in the time of the bubonic plague which is commonly known as the Black Death (the 14th century, where over 75-200 million died).
He wears all black, with a silver mask and green eyes. He appears to have some rats hanging into his belt, resembling something from the Black Death.
Plague holds his Contagion Crossbow around his wrist just like War.
The player can't headshot him.
The Lady tells you in the mission to "protect the people from a fate worse than death!". Ironically, the Last Herald of the Apocalypse is Death.