My guess is that the zerk was Traun. Here's why,
"When Alric sent word that a man was needed to pierce the corpse-city of Muirthemne to its dead heart, and obtain from it what might be the only hope for victory, Truan volunteered without hesitation."
-MythTFL Flavor Text
This fits the description. A single zerk goes into muirthenme's catacombs (its dead heart) grabs the head (the only hope for victory) and run like hell. Precisecly what he did. The rest of the people there were not with him as he went solo. They were just there waiting for him t arrive.
And for those Myth Quiz devotees, take another look at that intro cutscene. He's obviously not in the desert and he's obvoiusly got the head.
-Disc