Activate Altar of Dementia by sacrificing Gravecrawler, causing target player to mill cards equal to its power.
Cast Gravecrawler from your graveyard by paying ({B} magic symbol) or 2 life due to K'rrik.
K'rrik triggers, as well as each creature you control's extort ability.
Resolve the first K'rrik trigger, putting a +1/+1 counter on it.
Resolve the first extort trigger, causing you to pay ({WB} magic symbol) or 2 life due to K'rrik, causing each opponent to lose 1 life and you to gain that much life.