Emeric the Evil was a notoriously aggressive wizard who terrorised the South of England during the early Middle Ages, until he died prematurely in a duel with Egbert the Egregious (TBB).
He is the first known wizard to have supposedly owned an abnormally powerful wand made of elder. This is likely to be history's first detailed reference to the Elder Wand. When Emeric was killed in the duel, Egbert took possession of the wand (TBB, DH21).
Other canon notes and references
First year History of Magic students are taught about him at Hogwarts (PS8).
Students are described as confusing Emeric the Evil with Uric the Oddball (PS8); seeing as one was a notorious Dark wizard while the other was an eccentric who wore a jellyfish as a hat (FW), this could have resulted in some entertaining essays.