The link to video:
For example, and to start:
The Marine @ 1:43.
Name: Billy Bob
BB had recently joined the military, hence inexperienced. He had been deployed into the battlefield with other strong terran army when the zerg had come through their planet's shields. BB was not excited. BB regretted joining the proud Marines. In fact, BB wanted to go home. But there was no home, because the ferocious zerg were attacking his home world. So until the zerg came swarming to BB's deployed position, he waited, hopeful that the battle would be over soon.
Then suddenly, he heard a screeching noise with stomping steps, causing earthquakes from afar. BB readied his gun, but when he saw the zerg turn around the corner @ 1:04, BB lost hope, seeing the swarming zerg. He started shooting uncontrollably when they got close, and when the Nydus Worm tore through the concrete ground, BB looked behind him. He knew they could not do anything about the zerg. He stood there, hopeless and lost. He suddenly felt a shawdow of something massive above him. He turned back, and was instantly flattened like a pancake with bacon on top.
TL;DR (or short version)
Billy Bob lost hope in the battle and died.
So come up with your own and be creative!
It can be short or long with little to much detail.
For starters, you could do the Viking at the beginning, the Ultralisk, etc.