2019-08-23 : A player loses life if they’re dealt damage or if they pay life.

2019-08-23 : Greven checks for how much life you lost, not how much your life total has changed. If you lost 4 life but also gained 6 life, you still lost 4 life and Greven gets +4/+0.

2019-08-23 : The sacrificed creature’s power and toughness are determined by its last known information before it left the battlefield.

2019-08-23 : You sacrifice a creature (or choose not to), draw cards, and lose life all while Greven’s triggered ability is resolving. Nothing can happen in between, and no player may choose to take actions.