I steal a Necromantic Summons off my opponent. Then his creature dies. Necromantic Summons is unavailable to cast.
Then I shoot my own creature with Ajani Vengeant’s second ability. Necromantic Summons is unavailable to cast, but it should now be available to me, as the conditions for it’s casting have now been met (at least one creature in each graveyard).
//Edited Title - Tombstone