Anyway, I've come up with a small piece of fluff to go with the art piece.
The sisters of battle from the order of "The golden sun" were attempting to put down a slaanesh cult in the #removed# sector. For reasons unknown the order became corrupted by the slaanesh cult and proceeded to perform heretical rituals themselves
Not wanting to show the imperium that even Sisters could fall to chaos, the inquisition took a small force along with a company of Rainbow warriors space marines to destroy the sisters and put down the slaanesh cult for good.
After the cult had been destroyed and the sisters order destroyed. The inquisition left evidence that the rainbow warriors had gone traitor themselves and destroyed the sisters of battle.
The wider imperium is still blissfully unaware that even sisters can be corrupted and blame the rainbow warriors for destroying such devoted followers of the imperium.
The rainbow warriors to this day are trying to track down the inquisitor and prove to the imperium they are still loyal.