Rotsam

The rotsam can move through a space as narrow as 1 inch wide without squeezing.

A creature that fails its saving throw against the rotsam’s diseased bite attack becomes infected with the bog rot disease. Until the disease is cured, the infected creature’s skin breaks out in a rot-like rash that slowly spreads across its body, and its hp maximum decreases by 7 (2d6) for every 24 hours that elapse. After the first 24 hours, the creature’s skin starts to smell like rot, and creatures have advantage on Wisdom (Perception) and Wisdom (Survival) checks to find the infected creature. The reduction and rot smell last until the disease is cured. The creature dies if the disease reduces its hp maximum to 0.

The rotsam can climb difficult surfaces, including upside down on ceilings, without needing to make an ability check.

Melee Weapon Attack: +5 to hit, reach 5 ft., one creature. Hit: 5 (1d4 + 3) piercing damage plus 10 (3d6) necrotic damage, and the rotsam attaches to the target. While attached, the rotsam doesn't attack. Instead, at the start of each of the rotsam's turns, the target takes 10 (3d6) necrotic damage. If a creature ends its turn with a rotsam attached to it, the creature must succeed on a DC 12 Constitution saving throw or contract a disease (see the Bog Rot trait). The rotsam can detach itself by spending 5 feet of its movement. It does so after the target contracts its disease or the target dies. A creature, including the target, can take its action to detach the rotsam by succeeding on a DC 12 Strength check.