The Ur-Flan (more formally known as the Ur-Flannae) were a ethnic group related to the Flan people. Exact details of the Ur-Flannae are difficult to determine as they mostly vanished during the Great Migration.
What is known is that they were small in number, evil, and known to be powerful wizards. Feared, or at least respected, by their Flan kin, they were known to follow the teachings of Nerull in many regions (most notably Perrenland). They had a strong association with necromancy magic, many of whom later becoming undead.
Vecna was Ur-Flan before his ascension to godhood.