You can also kill them with any form of non-combat damage, even if that damage occurs during the combat phase.
Order only. Attach to a target unit you control. Action: When your capital is dealt damage, deal 2 damage to target unit in any corresponding zone.
Let's have an example:
I have a unit equipped with Asrai in Kingdom, a Spider Riders attacks my capital from quest and gives 2 damage. Now with the ability of the Asrai, the "corresponding zone" it's Kingdom, where Asrai is equipped, or the Quest of my enemy, where the damage where dealt?
We Smash, we Go, we Boom. And you?
V1.9 FAQ is out
Official printer friendly FAQ here.
My unofficial condensed rulebook here. I've started to include a couple extra clarifications in this, like the timing of when things are attacking, defending, etc, during the battlefield phase.
(If you have card questions, just make a thread about it, rather than posting it in this FAQ update thread).