Hounds of Tindalos

Hounds of Tindalos are beings that hunt down and kill time travelers. It is believed that they come after humans trying to alter the past, present or future and the course of fate. The hounds then hunt them down, tear them apart and disappear. The hounds can only manifest from angles, hence it is possible to, at least temporarily, hold them off by eliminating all angles in a closed room and replacing them with curves. The hounds can also be killed by iron bullets, bullets baptized in holy water and certain magical spells, but this does not prevent them from coming back. In most cases, the hounds leave a time traveler alone if they do not interfere with things.

Trivia

 * The "Hounds of Tindalos" originally appeared in H.P. Lovecraft's Cthulhu Mythos lore.
 * In turn, the original "Hounds of Tindalos" in H.P. Lovecraft's lore might be based off of hellhounds. The Outbreak Story's Hounds of Tindalos are also similar to Supernatural's hellhounds.