At the beginning of each upkeep, Tombostone Stairwell's first ability triggers, causing each player to create a 2/2 Zombie creature token for each creature card in their graveyard.
At the beginning of each end step,
Tombstone Stairwell's second ability triggers, destroying all tokens created by it.
When the token(s) die,
Embalmed Ascendant triggers that many times, causing each opponent to lose that much life and you to gain that much life.