The Roten Ritter is the Fire Mage brigade of Velthomer in Fire Emblem: Genealogy of the Holy War. Led by Arvis and composed of the finest knights from Velthomer, they consist mostly of Fire Mages, Mage Knights, and Mage Fighters. They are first fought in Chapter 5 of Fire Emblem: Genealogy of the Holy War and were aided in the execution of Sigurd and most of his liberation army in the Battle of Belhalla.
The term 'Roten Ritter' is German for 'Red Knights'. It should be noted that the grammatical correct spelling would actually be 'Rote Ritter'.