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The Plague of Shadows is the final boss of Plague Knight story, he's the fusion of Mona and Plague Knight essences.
After the defeat of The Enchantress, Plague Knight merged his essence with Mona, but the result was different from what he was expecting, The plague of Shadow spawned and attacked Plague Knight, defeated he transformed into the Corrupted Essence and fought again with the Knight, however he was defeated again, releasing Mona and turning into the Serum Supernus.
The Plague of Shadows appears as a darker version of Plague Knight and he even emulates all of his moves, as the Corrupted Essence he appears as a gigantic crow riding atop of a metal orb which resembles a cauldron or a furnace.
- The Plague of Shadows is similar to the Legend of Zelda villain Dark Link.