Your devotion to black times the number of opponents you have is greater than or equal to 15.
Steps
At the beginning of your end step,
Soulherder's second ability triggers, blinking
Gray Merchant of Asphodel.
Soulherder's first ability and
Gray Merchant of Asphodel trigger.
Resolve the
Soulherder trigger, putting a +1/+1 counter on it.
Resolve the
Gray Merchant of Asphodel trigger, causing each opponent to lose life equal to your devotion to black, and you to gain life equal to the life lost this way.
Activate
Unspeakable Symbol five times by paying 15 life, putting five +1/+1 counters on
Sage of Hours.
Activate
Sage of Hours by removing all +1/+1 counters from it, causing you to take at least one extra turn after this one.