First Banishment

‘It was one of the first missions the Grey Knights ever undertook. Three entire Brotherhoods and a quarter of a million Imperial Army soldiers set out to close the Damnation Cache.'
...
Many of them were of the lower orders, barely cogent things that served as foot soldiers for those who were especially blessed by their patrons, but one in particular was more formidable than any of the others. A greater daemon who served the Plague God, he tore through us with abandon and still we died even though the Cache had been closed. The thing was nameless to us so we did not have the power to banish him, but our combined psychic might was enough to bind him and seal him in one of the vast chambers. A chamber where instead of red Pythosian crystal, green gems studded the walls.
‘Of the three hundred Grey Knights who had set out from Titan to cleanse Pythos, barely twenty of us survived. The Imperial Army regiments fared worse, wiped out to a man during the final battle to subdue the Great Unclean One. With no athame to keep it permanently closed, it was decided to hide seven seals around the planet, each one different and only able to be undone in a specific order.'



Chapter Eleven