Of Tieflings and Mongrelmen

Whilst not naturally thought of together, these two races are, I posit, naturally connected. I hear your cries of disbelief already, but bear with me and all shall be made plain.

Mongrelmen, as we know, are a "mixture of many different creatures" and "lair in ruins, deserted buildings, or other places that humans once lived in or built" (emphasis mine). Tieflings, on the other hoof, are said to be "part human and part something else" and, whilst this is heavily insinuated to be something of the diabolic, demonic, or daemonic sort, it is not stated so in their description in the Planescape boxed set. This being the case...

The races known as 'tieflings' and 'mongrelmen' are, in fact, the same race. Originally human, their ancestors melded their bloodlines with many other races in an attempt to breed a new race with the best qualities of all. This plan may well have worked, if it hadn't angered the various gods of the different races, causing those gods to smash their civilisation and reduce them to savages. As savages, they quickly lost track of the old breeding plan and began to spiral into a vortex of chaos and mutation, though some are still born with wit, cunning, and magic power, the bulk of their race has become dimwitted and hideous caricatures of humanity.