Deformed Monsters in Sewer or Underground Dimension

by Mia

There was a movie I saw a long time ago about a sewer or underground dimension.

A guy's girlfriend is trapped in the sewer. His best friend becomes one of the deformed sewer creatures that ends up chasing him and his girlfriend around. The creature is black with a super long spinal cord for a neck (no flesh).

The friend / now creature tries to attack the other two until they show him something that reminds him of who he is (circular item), at which time he turns and goes after other monsters in the sewer, sacrificing himself so his friends can escape.