So, here is the cover I got for the publishing derby. I blocked out the pen name so it can't be linked to me, as it's supposed to be done without any links to known pen names. I have to write a story, 10K words or more, based on this cover:

So, based on the artwork, my mind is going in a few different directions. I think it's obvious that the figure depicted here is meant to be the last sentinel. What is he guarding? Is he keeping people/things/aliens from exiting or entering the portal? Is it more metaphorical? While on a phone it may just appear to be a dude on horseback with a spear, if you zoom in you can see some electricity arcing from the spear and some neon lights on the helmet. So this guard, the sentinel himself, is either someone from the future or perhaps an alien. I don't want to go with him being an alien, so I'll say he's a dude from the future. Maybe defending against aliens. I also noticed he's actually riding a unicorn, but I think that's dumb so I'm going to Photoshop the horn out of there. I saw nothing in the rules saying I can't slightly alter the artwork.
So, we've got a dude from the future with an electrified spear guarding a portal. He's the "last sentinel," which I think implies all the others are dead. It definitely looks like he's in some sort of wasteland with ruins surrounding him. I'm leaning towards him guarding the portal and trying to keep anything else from coming through it. Or more realistically, he doesn't just sit there and guard the portal all day (duh), but something at some point in time came through the portal and he is humanity's only hope. Hmmm. How about...
*dramatic music*
In the distant future, after sending countless signals into outer space in hopes of contacting some form of intelligent life, intelligent life instead found us. In a blinding flash of light, a peculiar mechanism appeared in the middle of (CITY), laying waste to everything surrounding. To everyone's amazement, alien beings poured through this portal. However, their intention was not to learn about our culture or share technological innovations; their intent was to subvert and conquer.
Their planet was drying up due to a runaway greenhouse effect and they sought to start anew on our ocean planet. Wasting no time, they obliterated the largest cities as soon as they arrived. Immediately after, they began enslaving the human race to use them as slave labor to build their soaring towers and disgusting technologies. Breeding plants were constructed, where humans were bred like lab rats and often experimented upon. The dead were used as food for the living. After only two years, the human population dwindled from more than fifteen billion to less than a hundred million.
The humans did their best to fight back against the Cthulhu-like aliens, but pink fleshy hands were no match for tentacled aliens that towered above their inferior human counterparts. The survivors lived in subterranean bunkers, scavenging for food and supplies for the entirety of their short lives. They lived in perpetual fear of death. The aliens were methodically discovering and eradicating all survivors. The human race didn't have much time left.
(NAME), who had worked as an electrician before the aliens arrived, had discovered their weakness and successfully defended the small underground enclave in which he and a few dozen inhabitants lived. They kept a small assortment of farm animals nearby for food and transportation; the aliens largely left the small-minded horses and cows alone and focused their efforts on eradicating humans, whom they saw as a threat. (NAME) could help craft devastating weapons and train the children to fight, but most of the time he just wanted to curl up in the bottom of a bottle and fall asleep forever, to forget any of this ever happened.
One night while he's scavenging the remains of (CITY), he comes back to find that his bunker had been attacked, and everyone captured by the aliens. They would likely be sent to the breeding plants or the slave camps. He found himself out of options. It was time to fight back. After a few days' preparations, he set off towards their stronghold.
(NAME) did not go into their lair to win, or to survive. He went into their lair only to kill them.
I think something like that might work.