Brown Evil Character - 4/8 - If an occupied Jerusalem site is in play, return all sites in battle to owner's territory. Jebusites cannot enter battle if King David or Joab (Hero) are in play.
I Chronicles 11:4-9
In order to have the effect I think you want, it should say:
"If an occupied Jerusalem site is in play, return all sites in battle to owner's territory. If King David or Joab is in play, negate that ability and withdraw Jebusites from battle."
Since the "cannot enter battle" phrase is equivalent to "is ignored" then here's what would happen:
I'm playing a brown site lock deck. I have King David or Joab in play. They attack. I block, since my ability isn't active yet. I am now being ignored by my own ability which gives me infinite initiative. I play any number of enhancements to get rid of any number of site access cards you might use. Assuming you don't have access to any LS's, you win the battle challenge by ignore, and never get initiative. You are by this time most likely locked out.