Stranded

The UNSC Skipper was on a routine combat mission when they were engaged by a independent cruiser. The UNSC took major damage and made a emergency slip-space jump out of the sector they were engaged in. However due to the damage the UNSC Skipper had to be scuttled and the crew abandoned ship over a desert moon hovering over a unknown planet. The marines landed on the planet using what salvage from the Skipper that they had available as they set up a distress beacon. As the marines tended to their wounded from the battle and continued to try make contact from any UNSC ships in the area they received weird reports from their scout teams they had sent out to make sure that the planet was empty. Then it seems a unknown threat crept closer to their base.

Soon the air was filled with the men on perimeter engaging a unknown enemy, as the shots from the perimeter guards rang out, their radio buzzed with activity. The UNSC Achlys had picked up their distress beacon and was en route to the moon to pick them up. However with the screams of soldiers in the air and the constant sound of gunfire, would the marines be able to hold on until help could arrive or would they fall to this unseen enemy.

Canonically: A few marines would survive this trial and evacuate the planet after their distress signal was picked up by the UNSC Achlys...however possibly at a cost.