Western Kingdom (The Messenger Series, #5)

Dargan does not know what happened, but he's officially dead.

Western soldiers appear outside the castle, saying, "This is a warning. We can appear anywhere, anytime. You will leave us alone."

Toby does not know who they are but gets a clue.

"Is í ding di féin a scoileann an dair."

A wedge of itself that splits the oak.