Originally Posted by Brother Lucian
Not an encounter Ive heard of, elaborate? Devastation of Baal will surely cast more light on the KDK involvement and plans.
The Fall of Shadowbrink was a tyranid invasion of a world that hosted a Grey Knight garrison. As more and more defenders were consumed, the GK were eventually forced to join the fighting until they too perished. This unleashed the Daemon they had been keeping sealed, which in turn sparked a full on daemonic incursion. The daemons were dismayed to find these aliens rather than the human quarry they had been expecting, and set about exterminating every tyranid they could find.
Initially the daemons' raw power swung the battle in their favour. However the description of the battle makes it clear that the daemons could not feed of tyranid deaths the way they could on other mortals, and the longer the battle went on, the more their power drained and momentum swung back to the tyranids. The tyranids saw the daemons not as prey, but as a rival predator competing for its prize. Between the warp static caused by the nids' Shadow in the Warp and the daemons' inability to feed off their foe, they were eventually defeated.
I will quote the paragraph that follows after a Greater Daemon of Nurgle was blasted apart by a brood of zoanthropes:
Originally Posted by Tyranid Codex, Page 30
With one of their number fallen, the remaining lords of the Abominatum realised the nature of the battle had changed. The Hive Mind was leeching their energies, severing the Daemons from the sustaining powers of the Empyrean. No real blood flowed for Khorne, just worthless alien ichor. As each rancid disease was unleashed by the children of Nurgle, so the next brood of tyranids had grown resistant to it. Without the fear or devotion of true mortals to sustain them, the Daemons were foundering fast.
It goes on to describe the eventual defeat of the Daemon horde, and the tyranids resume their feeding process. However this lore makes it clear that if Sanguinius' old rival had indeed engineered the tyranids' defeat, he did it solely to preserve the Blood Angels as his own prize as there is no other reward in fighting the tyranids as far as Daemons are concerned.