Gray Merchant of Asphodel in hand or in your library (on top of your library).
All other permanents on the battlefield.
Easy Prerequisites
Your life total is at least 6 if Gray Merchant is on the top of your library, or you have ({3} magic symbol) ({B} magic symbol) ({B} magic symbol) available otherwise.
Steps
Cast Gray Merchant either from your hand by paying ({3} magic symbol) ({B} magic symbol) ({B} magic symbol) or from the top of your library by paying 5 life.
When Gray Merchant enters, it triggers, causing each opponent to lose life equal to your devotion to black and causing you to gain life equal to the life lost this way.
Activate
Yahenni by sacrificing Gray Merchant.
When Gray Merchant dies,
Mortuary triggers, putting Gray Merchant from your graveyard onto the top of your library.