Mareilon was one of the ancient cities united by Entharion the Wise to form the Kingdom of Quendor in 1 GUE.
Mareilon was destroyed in 773 GUE when the Endless Fire was burned for 4 weeks.
The destruction of Mareilon caused Lord Dimwit Flathead to issue a series of 5,521 edicts that severely limited access to magic, and incidentally, lawyers.
It seems that the remnants of Mareilon was one of the lands that participated in the small conflicts and were to be united by Syovar under the Kingdom of Zork. In the days leading to the Conference of Quendor, Mareilon was one of the lands that used the truce to amass some armies, and in case the Conference broken up in acrimony, the Mareilon armada was ready to blockade the harbors of Frobozz. However the Treaty of Quendor was signed successfully.