The Deluge

-and the Lady will weep for our transgressions. She'll weep for days on end, unable to stop us from damning ourselves, and her tears will become a new ocean, one that drowns us all.

-Sermon of the Sea Church 

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Of the many churches, the Sea Church paints gods in kind blues and greens, as well as grim greys and blacks. While many of the clerics of that church speak highly of God's kindness as well as the bounty and healing that she provides, there is one tale that they bring out only in the most dire of circumstance. 
The Deluge. The end of the world. 
There are differences in what brings about the Deluge, and how exactly the world comes to the end, but the throughline is always the same. Humanity dissapoints God in such a way, worse than the acts that barred us from the Promised Garden, and seals our fate forever. Impiety, crime, heresy, and war. The monstrous actions that humanity can inflict upon itself and others. One, or perhaps all, of these things could bring us all to ruin. 
As for the Deluge itself, I think the name is fairly self explanatory. The rains fall, the sea rises, and humanity cannot escape the tide that chases them like hounds chase deer. All that is left will be the fish and the Titanic Fauna that ply the waters that swallowed even the mountains.

Within the Sea Church, it's generally considered a pretty sever admonishment to suggest that particular actions, especilly by an individual, would bring about the Deluge. Sever enough that for that particular church's faithful, it's nearly the harshest if not the harshest thing that can be said to another person, and has been the source of many fights, ironically.