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It's a story known as The Fifteen of Midael.
A group of fifteen Deathwatch marines on a strike cruiser are chased by a massive chaos warship through the warp, taking huge amounts of fire, and leading it back to Watch Station Midael where they drop-pod down just as their ship explodes and hold up for a siege in the watch station. They held out against large numbers of chaos space marines before the warship unleashed the warp spawn you so wonderfully portrayed. Half of the marines held the gate against the continuing numbers of chaos marines while the other half led the beast further into the base, trapping it in a room with them and activating ancient wards that ripped apart everything in the room. The warp spawn and the marines that trapped it all perished.
The other half had led a massive charge against the attackers, finally turning them back. However, when the chaos warship realized they had lost the ground war, they opened fire on the watch station and annihilated the remaining marines, and all of the surrounding area from orbit. The watch station survived, but its defenders did not.
When other members of the Deathwatch finally arrived, all that remained of the defenders were their weapons. Their bodies and armor were never found.