Archon of Falling Stars and Phyrexian Altar on the battlefield.
Steps
Activate Phyrexian Altar by sacrificing Archon of Falling Stars, adding one mana of any color.
When Archon of Falling Stars dies, it triggers, returning Necromancy from your graveyard to the battlefield.
When Necromancy enters the battlefield, it triggers, returning Archon of Falling Stars from your graveyard to the battlefield, and attaching Necromancy to it.