Some of my favorites below, feel free to chime in with your favorites in the comments...
"Till shade is gone, till water is gone, into the Shadow with teeth bared, screaming defiance with the last breath, to spit in Sightblinder’s eye on the last Day." The Aiel, Wheel of Time by Robert Jordan
"Night gathers, and now my watch begins. It shall not end until my death. I shall take no wife, hold no lands, father no children. I shall wear no crowns and win no glory. I shall live and die at my post. I am the sword in the darkness. I am the watcher on the walls. I am the shield that guards the realms of men. I pledge my life and honor to the Night's Watch, for this night and all the nights to come." Night Watch Oath, A Song of Ice and Fire by George RR Martin
"In brightest day, in blackest night,
No evil shall escape my sight.
Let those who worship evil's might,
Beware my power, Green Lantern's light!"
-Green Lantern Oath