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 Crookshank Kobolds to your hand from your graveyard.
Cast Crookshank Kobolds by paying ({0} magic symbol) .