In Fire Emblem: Genealogy of the Holy War, little is mentioned about the Deadlords, but they are all fought in the Final Chapter near Barhara and eventually defeated, including Mus himself.
Mus appears to be the leader of the Deadlords, as he is the only member to have Leadership Stars in Thracia 776 and is considered the map Boss in Awakening's DLC.
Mus is a Baron with a Silver Lance and the skills Great Shield, Pursuit, and Continue. He is thus average in terms of offense, as compared to the other Deadlords. However, as a result of his high Defense and Resistance stats, coupled with his Great Shield skill, he is thus difficult to defeat. It is recommended to attack Mus with Forseti, Book of Naga, or any other strong spell.
Mus, being the Deadlord clone of Raydrik, naturally inherits exactly the same stats as the former. As such, the Blaggi Sword will still be effective against him, and should be primarily used to defeat him. It is strongly advised to do this as quickly as possible, so as to prevent Mus' leadership stars from supporting the other Deadlords.
Eins, Mus' Japanese name, is a German number, which, in this case, means 'one'.
In the English version, the Deadlords' names are based off the Latin names of the animals of the Chinese Zodiac. "Mus" means "mouse" or "rat", the first animal of the Chinese Zodiac.