The blood of the gods.

The grand fight between good and evil already happened, and nobody won. The gods were so evenly matched when they went to war that they utterly unmade each other.

However, the nature of gods being what it was, eternal in their power, while the gods themselves are dead and gone, their power is not. It's just changed, corrupted.  Even the names of the gods are forgotten by most, though a few races hold on to their names, and some even continue to worship in secret, in the hope that they someday return.

The power and blood of the gods was irredeemably corrupted in their destruction and death.  The final blow that destroyed all of the gods was a seed of corruption, essentially a magical virus of unbelievable power, that turned their own power and blood into poison and corruption that unmade them all. As a result, the pools and remnants of their power that remain are impure and corrupt to this day. There are those who use it, but they inevitably become a reflection of that corruption.

Each and every one of the intelligent races on this world were originally created by one or another of the gods to use as tools to expand their wars upon each other. Thus, until the gods died and the war ended, the races were all slaves, who knew nothing but war.  All of the arts of man, aelth, duer, etc. are thus either of recent invention (recent being a relative term, or course) or are derived from the tools of war given them by the gods.  The Aelth were the first such race created, and thus have the longest history, albeit most of it is of servitude to the gods.   Similarly, the oldest ruins, dungeons and suchlike, are forgotten things from the age of war and god-death.