Large Giant, Chaotic Evil
Plague knights are especially degenerated rot trolls that are bolted into massive suits of heavy armor before being compelled into service to the march of death. These particular creatures are further engineered such that their bodies produce and carry horrific gases that indiscriminately infect all life they touch. Unlike their undiseased kin, rot trolls do not have natural regeneration. Though their physical bodies continue to mend, it is only to facilitate a constant process of decay and degeneration that corrupts the very air surrounding them.
Magic-Infused Weapons. The power coursing through the knight's veins flows into its weapons. The knight's weapon attacks are magical. When the knight hits with any weapon, the weapon deals an extra 2d6 necrotic damage (included in the attack). Siege Monster. The plague knight deals double damage to objects and structures. Rancid Degeneration. At the end of each of the plague knight's turns, each creature within 5 feet of it takes 11 (2d10) necrotic damage, unless the knight has taken acid or fire damage since the end of its last turn.
Multiattack. The knight makes one Bash attack and two Maul attacks. Bash. Melee Weapon Attack. +8 to hit, reach 5 ft., one target. Hit: 11 (2d6 + 4) bludgeoning damage, and the target must make a DC 16 Strength saving throw or be knocked prone. Maul. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 18 (4d6 + 4) bludgeoning damage plus 7 (2d6) necrotic damage. Virulent Miasma (1/Day). The knight releases toxic gas in a 30-foot-radius sphere centered on itself. Each creature in that area must make a DC 18 Constitution saving throw, taking 14 (4d6) poison damage on a failed save, or half as much damage on a successful one. A Humanoid reduced to 0 hit points by this damage dies and rises as a zombie (see its stat block in the Monster Manual) 1 minute later. The zombie acts immediately after the knight in the initiative count.