The Endless Fire exhibit at the Inquisition's Perils of Magic exhibit in 1067 GUE
The Endless Fire was started by a civil servant on 18 Estuary, who thought he was casting the ZEMDOR spell ("turn original into triplicate") but who, instead, cast the ZIMBOR spell ("turn one really big city into lots of tiny, little ashes").
It raged for 4 weeks and destroyed the city Mareilon in 773 GUE.
When its origins were discovered, Lord Dimwit Flathead issued a series of 5,521 edicts over the following few weeks, which had the effect of severely limiting access to magic, and lawyers. All magic was entrusted only to the various Enchanters' Guilds.