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,
South Wind Avatar's first ability triggers that many times, causing you to gain that much life.
South Wind Avatar's second ability triggers that many times, causing each opponent to lose that much life.