The idea I came up with was an activation you could use in your hand
The most natural comparison is the Double Faced Cards in Zendikar Rising that were either spells or tapped lands, but when you played DFCs you could have cards with a range of casting costs, which meant that youd typically want to cast some of them in the early to mid game
When Necromancy enters the battlefield, if it's on the battlefield, it becomes an Aura with "enchant creature put onto the battlefield with Necromancy." Put target creature card from a graveyard onto the battlefield under your control and attach Necromancy to it
Pyramid Turtle is absolutely insane, and it is awesome
The Enchanted variant shows a more triumphant pose atop a rock, with a songbird perched on the uplifted finger of a smiling Snow White
Not to mention that, when you add this card to other ramp decks, you can cheat this into play, or cast this 8-mana spell way ahead of curve