Disease Elemental (5e Creature)
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Medium elemental, chaotic evil
Skills Perception +4
Disease Form. The elemental can move through a space as narrow as 1 inch wide without squeezing. A creature that touches the elemental or hits it with a melee attack while within 5 feet of it takes 3 (1d6) poison damage. In addition, the elemental can enter a hostile creature's space and stop there. The first time it enters a creature's space on a turn, that creature takes 3 (1d6) poison damage.
Disease Relocation. The elemental knows the location of all diseased people within 2 miles from its location. As an action, the elemental can teleport to the location of a person infected with sewer plague; it must complete a short or long rest to do so again.
Multiattack. The elemental makes two slam attacks.
Slam. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 14 (2d8 + 5) bludgeoning damage.
Infectious Whirlwind. The elemental forms into a tornado of disease and plague. All creatures in a 10 foot radius must make a DC 15 Constitution saving throw. On a failed save, a creature takes 16 (3d8 + 3) poison damage and becomes infected with sewer plague, showing symptoms immediately; a creature which is already infected instead gains 1 level of exhaustion. On a success, the creature takes half as much damage and is not infected.
Life Drain. Melee Weapon Attack: +2 to hit, reach 5 ft., one target. Hit: 13 (3d6 + 3) necrotic damage, and the target is grappled (escape DC 16). At the start of each of the elemental's turns, creatures it is grappling take 13 (3d6 + 3) necrotic damage and must succeed on a DC 15 Constitution saving throw or become infected with sewer plague, showing symptoms immediately; a creature which is already infected instead gains 1 level of exhaustion.
A disease elementals is created out of death and plague. Imagine a great trading city, now ravaged by the plague; the piles of bodies and the smell of rats and flies forms a cloud of noxious gas, and after a few days an elemental forms with the mind of the disease. They wish only to infect the healthy and bring death. As long as there is disease, there will be disease elementals.
Variant: Other Diseases
The stats above are for an elemental of sewer plague. To make a disease elemental for a different infection, change the effects of the elemental's infectious whirlwind and life drain as such: creatures which fail their first Constitution against the attack are infected with the appropriate disease and display symptoms immediately, and creatures which are already infected take whatever cumulative penalties or conditions the disease would inflict over time. For instance, a creature which fails this save while fighting a sight rot elemental would be infected with sight rot, or take a −1 penalty to attack rolls and ability checks that rely on sight if already infected. These changes may increase the elemental's CR, depending on which disease it is based on.