I'm not really familiar with planescape, so any simulatities to it aren's intentional.
Rather, the premise is:
- the Gods lost the Dawn war. They are all dead, though some have given part of their essence to mortals, so that they can be gods of individual shards,.
- in loosing, they shattered the world, to prevent the primordials from destroying everything.
- the various races ended up on various shards.
- when the shards are connected without the proper precautions, energy spills between them and mutates people on either side.
- many of the races are "hybrids" in this way: half elves = human + elf, tiefling = human/elf + demon, gnoll = goblin + demon, orc = goblin + human/elf, drow = elf + shadowfell, gnome = halfling + feywild, spriggan = halfling + demon... and the list continues.
- daemons are the primordials soldiers, tasked with destroying all the shards.
- devils are the soldiers of gods who died, abandonned to make there own way, but still tasked to unite the shards.
- the shards have been loosing to the demons, and as a result, a bunch of races are collected on one shard (the PC's starting place). This shard was the home of the goblinoids, who aren't happy to have been displaced.
So, as I said, a home brew. Though the PCs (about to hit paragon) don't really know most of this... this is all the deep background, which they will be discovering over the next tier.
That said, they have already experienced the mutational affects of shard interaction - the eladrin paladin is now a tiefling after blocking a portal to one of the demon shards.