({BP} magic symbol) plus either ({WB} magic symbol) or ({BP} magic symbol) available.
Steps
Cast Mourning by paying 2 life if able, or ({B} magic symbol) otherwise.
Each creature you control's extort ability triggers.
Resolve the first extort trigger, causing you to pay 2 life if able, or ({WB} magic symbol) otherwise, causing each opponent to lose 1 life and you to gain that much life.
Repeat step 3 for each remaining extort trigger.
Resolve Mourning, attaching it to a creature you control.
Activate Mourning by paying 2 life, returning it from the battlefield to your hand.
Repeat until an opponent loses the game due to having 0 or less life.
Notes
Once you have two or less opponents, the combo no longer makes enough life to continue. You may repeat the loop using your own resources as desired.