The World of Ysat'naf

 A bog-standard fantasy world for dungeon-y, dragon-y fantasy systems.

Use all the same fantasy assumptions those sorts of games bring to the table with one major exception: all alignments are inverted. (So chaotic becomes lawful, good becomes evil, and the like. Neutral alignments stay the same, as do unaligned alignments if you're using 4e or 5e.)

We'll keep it straightforward with two simple rules:

1) All extraplanar creatures summoned to Ysat'naf have their alignments inverted automatically. (Depending on how the Game Master interprets things, this may or may not change their core programming. It will at the very least change how they attempt to complete their goals and what sorts of behaviors they attempt to instill in the creatures they encounter.) Why this happens is not terribly clear, and finding the source of this phenomenon would obviously make a good goal for player characters to achieve. (I'm thinking it's some sort of curse upon the land, or maybe some kind of magical radiation that suffuses the atmosphere.)

2) All creatures native to the world also have their alignments inverted, although there is no supernatural force backing this up. (We can casually surmise that this inversion is due to the influence of extraplanar visitors who have had their alignments inverted and then proceeded to proselytize among the native populations.) As such, individual members of the various fantasy races can have any alignment and you might have the occasional "rogue" good elf or evil orc, but populations as a whole tend to stabilize to the opposite of their bestiary-listed alignments. They can retain their well-trod personalities and niches, now recast through the lens of their new alignments, so you get Lawful Evil elves in a strict hierarchy whose aesthetic predilections come across as Toreador antitribu-like displays of blood-daubed wicked decadence — like a whole race of Hannibal Lecters — while Lawful Good orcs now fulfill the Proud Warrior niche, a Klingon-like Golden Horde trying to throw elven imperialists off their land.

To fully embrace the potential Mirror Universe sensibility of such a place, it does not affect most Prime Material natives who might travel here by extradimensional gate or what have you. (So your characters are unaffected when they come through, although their familiars and summoned allies might behave very differently if they originate from another plane.)