You control a creature with power nine or greater.
Steps
At the beginning of your upkeep, Mayael's Aria triggers, causing you to put a +1/+1 counter on each creature you control and gain ten life.
At the beginning of your end step, Lathiel triggers, putting ten +1/+1 counters on a creature you control with power ten or greater.
At the beginning of your next upkeep, Mayael's Aria triggers, causing you to put a +1/+1 counter on each creature you control, gain ten life and win the game.
Results
Win the game at the beginning of your next upkeep.