Guardian

Potential Spoilers Ahead.
Guardians are creatures (perhaps machines) that can only be found in the Depths, and are the last challenge you'll meet before getting the ending. They look like tall black statues with arms with beads and tentacle-like legs stuck to the ground, also with a X-mark (the symbol for the max karma level). Their behaviors towards the slugcat depend on your current karma level. If you have karma level X, they will ignore you even if you walk past them. However, if you have any karma level below, their X symbols will glow red and they'll force you back to the entrance. Try too many times and they will kill you.

Trivia

 * There is a video in which a player bypasses the guardians with what is considered to be an exploit with low karma level by consuming Mushrooms and throwing two Flashbangs. This video is currently the Any% world record for beating the game.