The Sewer Spirit appears as a flat black silhouette of a hand that comes out of a sewer's manhole.
The Sewer Spirits only appear in certain manholes from the many that are scattered around the Town. If the Protagonist were to walk on top of its manhole, it will come out and grab the player. It can't do any direct harm, but it will render the Protagonist immobile while keeping the Heartbeat at high speed, potentially trapping the player long enough for other hostile spirits to attack. It will let go after about 5 seconds