Murtagh is Morzan's son, one of the Forsworn. Murtagh flees from Galbatorix's castle and grows up as a warrior, tracking down the Ra'zac and Galbatorix's troops. After the attack in Tronjheim in Book 2, Murtagh is captured by Galbatorix and is forced in the ancient language to sware never to betray him and he is given a dragon named Thron. Murtagh becomes an evil Rider.
Yes, Murtagh was Eragon's half-brother.
John Murtagh was born in 1943.
Elaine Murtagh was born in 1940.
Johnny Murtagh was born in 1970.
Brendan Murtagh was born in 1983.
Tim Murtagh was born in 1981.
Chris Murtagh was born in 1984.
Andy Murtagh was born in 1949.
Murtagh believed he was telling the truth. Eragon and Murtagh really are half-brothers and share the same mother. The part Murtagh was incorrect about was who Eragon's father was. Murtagh's father was Morzan, whereas Brom was the father of Eragon. Eragon was not even aware of this himself until after Brom's death.
Murtagh is certainly not evil. He is just being controlled. Galbatorix has complete power over him, and keeps Murtagh as a slave.
Arthur Forde's birth name is Hanna-Forde, Arthur.
Brinsley Forde's birth name is Brinsley Allan Forde.