The Deadlords (暗い武将, Dark Warlords in the Japanese versions) are the 12 undead warriors that serve the Lopto Sect and the Grimleal. The Deadlords trace all the way back to Emperor Galle, although it appears that the Deadlords are not the same individuals each time they are summoned.
In Fire Emblem: Genealogy of the Holy War, very little is said about the Deadlords at all, but they are fought in the Final Chapter alongside Julius. The best way to defeat them is to either lure them away from Julius and have your entire army defeat them at once, or you could defeat them mostly with Julia with the Book of Naga and with your Forseti user.
In Fire Emblem: Thracia 776, the Deadlords are explained a little more, but only six of them are seen, which are: Mus, Bovis, Tigris, Draco, Canis, and Porcus. After Raydrik is killed, he is remade into the Deadlord Mus, and if Eyvel is not saved, she is remade into the Deadlord Draco. Also, Galzus, Sara, and Lifis will become the Deadlords Bovis, Canis, and Porcus, respectively, if they die or are not recruited, and Dagdar will become the Deadlord Tigris if Chapter 8x is skipped. Thus, it is learned in this game that Deadlords are "born" by the Lopto Sect turning other people into them, and both Manfroy and Veld seem to have the ability to do this.
The Deadlords appear in Fire Emblem: Awakening in the main story and in the DLC episode Infinite Regalia, although their origins are shrouded in mystery. They appear during the main story in Chapter 22, (An Ill Presage). They appear to be aiding Aversa (although none of them say anything throughout the chapter). Five of the warlords wield holy weapons of Jugdral while the others wield Brave weapons.
In Infinite Regalia, 36 Deadlords appear (although there are only 12 unique ones), wielding a variety of powerful and legendary weapons spanning from Akaneia to Tellius. These Deadlords appear to have retained some degree of their humanity, and even go as far to express sadness when Chrom and his companions leave them. They proclaim to Chrom that they are warriors who fell in a battle with the world at stake. Some of the characters express a mysterious sense of familiarity with the warlords, suggesting some of them are Chrom and his companions who fell in the battle with Grima. Due to the large variety of weaponry, it's possible the warlords also include fallen warriors from other continents.
The Deadlords, their class, and other information are: