The solar system of Toril (Realmspace) from Forgotten Realms has been transported into a backwater region of the Imperium of Man. What happens next?
The warp is not directly accessible inside of the Toril System, but the D&D planes are still accessible via planeshift, gate, etc. This means there is now a conduit from the D&D Planes into the 40k galaxy, and potentially a conduit in the other direction if 40k characters could learn to master D&D spells.
D&D Arcane and Divine magic does not work if you travel too far from Toril. Practicing wizardry in nearby systems and worshipping Toril dieties can extend the weave, increasing the region where D&D magic can function. Likewise, warp phenomena could occur on Toril if enough inhabitants begin worshipping Chaos. The same applies for other 40k phenomena such as the WAAAGH.
Since the Imperium is a much larger entity, let's level the playing field (at least a little) by saying that the Imperium is not immediately aware of the appearance of Toril, but that the poweful NPCs of Toril are aware that Toril is now inside of a different and dangerous galaxy.
I got some inspiration form this thread: forums.spacebattles.com