I didn't play the divinity games, but what you wrote there resonates with the caption on this board : "In the silence of a godless age, the faithful are cleansing the world in the hopes of reclaiming divine favour. But from the embers something unseen begins to awaken."
So do you think they picked the wrong guy to sacrifice or that their ritual was noticed by a different being than they hoped for? Or maybe that they didn't really know whom they were sacrificing to? Or maybe it had something to do with the central dwarven character who becomes the centerpiece of the ... thing?
Well... Given the way the gods work in Divinity, is that they are Eternals who feasted upon the Source in the Veil - Which is the space between Rivellon and the Void.
Which is when they overthrew the God King and banished him to the Void.
If the age is godless... Then it presumably takes place post OS2 (Where the gods were destroyed both by Lucian actively draining their Source and also a big fight with the Godwoken) - Though the existence of orcs doesn't track with that as they weren't a thing by the time OS2 happened.
Well, whatever the time is... If they wanted to "Reclaim Divine Favour" then they'd be looking at tapping into the Source. Likely by sacrificing Sourcerers, which is not a particularly new revelation.
However, messing with the Veil by manipulating Source (I.e. Their whole "Murderize the sourcerers" thing) messes with its integrity... Meaning as they get closer to the Veil to try and reignite a Source powered God... Then the closer they get to the Void, where the God King is eager to return to the realm. This has been a continual theme throughout Divinity games, Sourcerers using Source magic will attract Voidwoken whom are thus able to cross over from the void.
So Bernie here is most likely a Sourcerer. Whom is being ritualized in an attempt to have his Source be able to fuel one of the Gods. Only, by unleashing his Source power, what they've done is given an avenue for the God King to break through into Rivellon (Again).
Now, why this seems to catch them off guard, is either because Bernie happened to be an extra powerful Sourcerer, so he had more power within him to weaken the Veil and allow the Voidwoken to breach into the world, the collective power of Sourcerers overall happened to build up to a point where a breach was possible, or simply it took time for the God King to notice/find the cracks being made by the sacrifcing of Sourcerers and this happened to be the first one he capitalized on.