I read it sometime around 2010, it involved a young man, possibly a teenager that fought using a spear, and at some point the modern world was taken over by demons, and the nations of the world negotiated with the demons to survive. He fought as a gladiator in an arena filled with demons. It was part of a series, and for some reason I keep thinking of the spear of Cucuhlainn.
This is The Tapestry Series by Henry Neff.
The Hound of Rowan Bit "Harry Potter"-ish. With the set-up of the Rowan School and his adventures there.
The Second Siege Where the demons take over.
The Fiend and the Forge This one has the gladiator bits.
The main character, Max, is a throw-back hero with special powers. Often called the Hound of Rowan, he is a spiritual Cu Chulain and does get a spear at some point (can't remember if it is Cu Chulain's).