Sorry parik.uhlemann, for making it a confusing chapter, here's a summary:
Harry and Percy went to the graveyard.
Voldemort rose.
Voldemort tried to kill Harry.
Percy jumped in front of him.
Percy was hit by the Avada Kedavra curse.
Really sorry if it was confusing!
Also, if there's anything that Percy does that confuses you, you probably don't remember some moments in HOO, but they will be explained much later, in my second book after this.
When Percy muttered "I promised her." He was thinking about using blood bending to stop Wormtail from threatening Harry, but he promised Annabeth that he wouldn't.
Also, the gods told him not to interfere with the rising of Voldemort, as the Fates decreed that he would need to rise to be defeated.
All bow/curtesy to Percy.
Peace out.