Xanathos Event

Not even 70 years ago the world was a much different place. The status quo was catastrophy constantly being thwarted by heroes. So often and prevalent were threats to the fabric of the multiverse and existence as we knew it that it became common news, and the more capitalistic of heroes started charging for their services. For centuries there were businesses built on doing heroic work, and the bosses of these businesses were heroes themselves of such renown they were often invited to the dinner parties of deities.

Around 70 years ago that all changed. A Centaur Bone Knight named Xanathos unleashed an army of the undead the size of which made even the gods of the dead halt and take notice. It was lead by one of the last remaining original Vampires, and it threatened the world. But this was merely a diversion; as the heroes of the world attacked this army of bones, on the other side of the world Xanathos made use of a magic so evil that demons and devils balk at the mere idea of it. In doing so Xanathos’ power became mightier than any god, and he changed reality.

The borders between worlds were repaired and then doubly bolstered; going between the realms now requires immense magical strength where before any ritual would do. Magic itself has been rewritten, and many spells now no longer work and ancient tomes are sometimes useless as the knowledge they have is outdated. High Magic, once common place, is now known only in legend.

An entire continent is devoid of all life, and venturing too deep turns things into dust. The Cursed Continent is where Xanathos wrought his magic, and nothing stirs from there, not even the dust that blankets it. Xanathos himself seems to have succumbed to the magic he called upon, as even the god he once worshipped cannot sense him.

The Aftermath
After the Xanathos Event, wars broke out. There were many wars, all over the place, once a great deal of people realized that the magic that either held them back from invading different locations or kept them from improving their station in life... suddenly didn't exist any more. Some old kingdoms survived. Some new countries were created, mostly because the newest guy with the army at his back said so.

Despite the fact that magic had been re-written, magical beasts still exist.