Relics is a short story by James Swallow, first published in Inferno! magazine #43, and re-published online in April, 2013.
Out on the unforgiving desert plains of the planet Serek, a squad of determined novices of the Doom Eagles Chapter are undergoing a brutal training regimen. Gruelling in the extreme, some of these would-be Space Marines are struggling in the harsh conditions. But these trials are nothing compared to the danger that comes for them from beneath the sands when the blood and sweat of the training ground suddenly becomes that of a deadly battlefield...