Nightblade

Nightblade - Ryan   Kirk Oriental themed fantasy. The Kingdom split into the Three Kingdoms many years ago, many believe it was caused by the nightblades, secret assassin/ninja warriors with more skills than the average soldier.
A splinter group calling themselves the dayblades thought they had seen the last of them.
Into this world come three children, some with the nightblades power.

They face danger and hardship from the beginning, learning things about themselves as they are brought up in different and opposing environments.

This is a good book, showing how actions that seem good can have extreme consequences. As the main characters are brought up in different factions, it lends an air of ambiguity to what they are taught. Neither side is necessarily bad, but work from the perspective they have been given.