Simply, The Empire. The Empire is ruled by King Galbatorix, and is directly opposed by the rebel group, the Varden (literally, "Warders"), and indirectly by Surda, which is a state which seceded from The Empire, and watched over by King Orrin.
Surda is considered a minor threat by The Empire, so it has not yet been invaded. However, they are the ones who have been secretly funnelling supplies to the Varden.
Eragon
The Empire's army, the Elven army, the Dwarf army, the Varden, the Surdan army and the Urgals.
Eragon was played by Ed Speleers.
The conflict is when Murtagh comes to the battle between the varden and the empire - he is not dead - he is a dragon rider sworn to King Galbatorix. His dragon's name is Thorn. He kills King Hrothgar and battles Eragon and Saphira and defeats them, but Eragon sees the heart in Murtagh and convinces him to let Eragon and Saphira go. Before Murtagh leaves, he tells Eragon that he is his half brother, making Eragon Morzan's son. He then takes Eragon's sword and retuns to Uru'baen. Roran kills the twins - also alive and evil - and the Varden defeats the empire. Read on in Brisingr, the third book, for more.
The princess elf from Eragon was named Arya.
The actor that plays Eragon is Edward Speleers.
Eragon's true name in the book Brisingr is Eldarion.
The sword is Brisingr forged by Rhunon through Eragon. *It lights up in flames when Eragon says its name.
he heard it from the townspeople or rumors about a new rider.
Garrow.
Alagaesia
Eragon