A/N: I wrote this whole thing as a joke. What would a space marine think of the Gods of Mount Olympus, demigods etc.?
I haven't read the Percy Jackson series in about 5 years, so do review or PM me if i've got something wrong.

"Suffer not the unclean to live!" I bellowed, amplified over the battlefield, as I sliced a warp-spawned cultist's head off with a spray of polluted blood. It was just too easy.

"Servants of the false emperor!" My head snapped up. A damned Marine sorcerer, a dabbler in unholy Warp magicks! My fury and bloodlust peaked, and I activated my jump pack, meaning to land next to him and sever his foul head from his body. "You shall be damned into the Warp!" he cried, cackling with insane laughter, and as I flew over the heads of my battle brothers all I saw was his outstretched hand, a fleeting flash of incandescent lightning, then nothing.

Black.

I do not know how long I floated in the immaterium, but all I know is that it must have been the Emperor's hand that guided me out of it, for suddenly the blackness disappeared, and the metal boots of my power armour landed on hard tarmac, the servos whining as they compensated for the fall. I looked around in confusion- I was on no world I recognised. A collection of blocky buildings, like bog standard Imperial hab-units, but less practical, was surrounded by lush, green vegetation. No obvious stains of Chaos- it didn't look like a death world. However, my analysis was cut short by a scream, and the wall closest to me exploding outwards in a shower of mortar and rubble. A small humanoid emerged, stumbling and coughing from the hole, followed by...

"Daemons!" I snarled. "Warp spawned pieces of filth! You shall not escape the Emperor's wrath!" I sprinted towards the creatures, my feet punching holes in the flimsy tarmac, firing my bolt pistol with precise shots even as I barrelled forwards. Two shots, two kills. I thumbed the activation stud on my chainsword just as I reached the monstrosities, which had stopped chasing the child, and turned in confusion. I swung the chainsword in a precise arc which decapitated the first monster with a squeal on chain-teeth. The second one in line was dispatched with a thrust through the chest, spraying ichor over the tarmac, spattering the child who was standing at the edge of the tarmac square with a look of utter terror and shock on its face. There were no more daemons out in the open, but I heard a feral snarl from the hole in the side of the building, so I ducked inside, forcing my way through the hole, coming face to face with another pack of the beasts, which were heading for the hole, presumably trying to get at this child. The first one had only time to howl in shock before its head was blown into pieces by a speeding bolt. I pumped the trigger of my bolt pistol, crosshairs appearing in my vision as I targeted each daemon, blowing several of them into bloody chunks. The magazine clicked empty, bun just as I ejected it and reached to fit a new one in, a daemon dropped from the wall above the hole where it had been hiding, and attacked me with a speeding ball of fire.

The fireball burst on the ceramic backplate of my armour, triggering a couple of warning lights inside my helmet, as I spun round and broke all the bones in its face with a vicious head butt. It stumbled backwards, howling, as I kicked it through the wall, the servos in my armour giving my kick unnatural strength. It twitched, then laid still, ichor spreading out of its body in a puddle, melting through the tarmac. I heard a daemon approaching me from behind, so I once again activated my chainsword, sweeping it around in an arc that sawed it in half, the severed pieces falling to the ground with wet thuds. I barrelled into the next creature, noticing that two humans had entered the large hall I was in from the other end. The creature let out a bestial roar, but I swung the bolt pistol into its face, pulping it. I sawed the head of the monster in half, brains flopping out onto the floor with a wet splat. I looked around, chainsword at the ready, but no more monsters were left inside. I reloaded my bolt pistol, and prayed to the Emperor there were more daemons to slay in his name, when I heard a faint, pitiful scream from outside. Evidently, my prayers were about to be answered.

I strode back to the large hole in the wall, aware but uncaring of the two humans following behind me. These daemons really were of the more pitiful kind. Some took the form of oversized daemonic dogs, some snake- women, clearly disciples of Slaanesh, and some the form of oversized, blundering men with the ability to conjure up balls of warp-fire. No challenge to a mighty warrior of the Adeptus Astartes. I forced myself out into the daylight of the planet's single sun, bringing more rubble down from the hole. At the far end, beyond the tarmac square, the child had backed up against a tree, surrounded by dog-daemons. I grinned, crosshairs appearing on my vision on the dogs. I brought the boltpistol up and fired, my massive arms and power armour absorbing the recoil. The bolts flew at supersonic speeds, easily penetrating the thick hide of the dogs, detonating and blowing large holes in the daemons, their corrupted blood splattering the ground. Five died instantly, a sixth was left whimpering and snarling on the ground as its lifeblood drained away. I activated my jump pack, flying over the pitted ground and landing amidst the remaining daemons that were backing away sheepishly, my descending boots staving a skull in, jellied bits of brain flying upwards and coating one of my pauldrons with a thin layer of filth. "Cleanse! Purge! KILL!" I roared, hacking the daemons to pieces and returning their cursed souls to the warp. I heard another scream-a daemon had used the messy deaths of its comrades as a distraction, and it was barrelling towards the human child, intent on disembowelling it. I knew I had no more shots in my bolt pistol magazine, but as I prepared to activate my jump pack, I felt a burst of psyker power behind me, and a whistling wind picked up the daemon and hurled it against a tree, breaking its back with a loud snap. I turned around slowly, slotting a new magazine into my bolt pistol, and aiming it at the head of the human who was now clambering through the hole tiredly. A psyker! Either he developed his powers by himself, something I was doubtful of considering his control over them, or he had them from the vile Chaos Gods, Slaanesh, Tzeentch, Khorne and Nurgle. He didn't have the stink of a Nurgle worshipper, nor the bloodthirstiness of an apostle of Khorne.

"Do you serve the false Gods of chaos with your warp-spawned powers, heretic?" I commanded, voice booming out of the armour's vox-unit.

"Gods? Chaos?" he asked, looking perplexed.

"You know which Gods I mean, heretic! Do you serve the ruinous powers?" I asked him, finger itching to pull the trigger and end the puny heretic's miserable life.

"The only Gods I know are the ones on Mount Olympus..." he said, his voice trailing off.

"Mount Olympus? Is this some place of heretical worship? Is this where you sacrifice to your foul Gods? You have the feel of a devotee of Tzeentch."

"Tzeentch? That's not a God, certainly not on Mount Olympus... what does Tzeentch do?" asked the person, looking curious and nervous at the same time, as my bolt pistol was still aimed squarely at his head.

"You don't convince me with your pathetic pretence, heretic. Confess of your sin, and I will bless you with a quick death."

"Seriously," spoke up the other human, a female who I hadn't been paying that much attention to, "we don't know of any Tzeentch, or of Gods of Chaos. Do you mean the Titans?"

"Titans? They are no Gods! They are giant machines of battle, armed with weaponry to deliver the Emperors wrath on xenos, mutants and heretics!" My voice grew louder, as I educated the primitive humans on the war machines of the Emperor.

"The Emperor? Which Emperor?" inquired the girl.

I was shocked speechless for a second.

"Is he like these Chaos Gods?" she asked me.

"HERESY!"

A/N x2: The girl's meant to be a demigod who hasn't developed her powers. Just for the record. Review pl0x!