From Destinypedia, the Destiny wiki
During his expedition beyond the Solar System, the Drifter and his crew encountered strange alien creatures on a desolate, icy planet. These creatures projected a field which suppressed Light, and were described as headless, "vacuous," shadowy forms. Many were found imprisoned within monolith-like alien structures, apparently in a form of cryogenic suspension, but others were also found roaming free around the planet.
As a result of the Light-suppressing field they generated, Guardians that died within close proximity of these creatures were incapable of being revived by their Ghosts, resulting in the Guardian's permanent death.