When Ruxa dies, Nim Deathmantle triggers, causing you to pay ({4} magic symbol) to return Ruxa from your graveyard to the battlefield, and attach Nim Deathmantle to it.
When Ruxa enters the battlefield, its first ability triggers, returning Kobolds of Kher Keep to your hand from your graveyard.
Cast Kobolds of Kher Keep by paying ({0} magic symbol) .