Cast Necromancy by paying ({2} magic symbol) ({B} magic symbol) , returning Leonin Relic-Warder from your graveyard to the battlefield, and attaching Necromancy to it.
Leonin Relic-Warder enters the battlefield, exiling Necromancy.
Necromancy leaves the battlefield, causing you to sacrifice Leonin Relic-Warder.
Leonin Relic-Warder leaves the battlefield, returning Necromancy from exile to the battlefield.
Necromancy enters the battlefield, returning Leonin Relic-Warder from your graveyard to the battlefield, and attaching Necromancy to it.