If Mundo is at 12 power on a battlefield and Unchecked Power deals 12 damage to all units at that battlefield, does Mundo survive due to gaining +1 power when Unchecked Power goes to the graveyard?

Ruling: Yes, Mundo survives. Unchecked Power marks 12 damage on Mundo and moves to trash (triggering Mundo's passive +1 power) before the cleanup occurs that would check for lethal damage.

Sequence:

• Unchecked Power resolves, marking 12 damage on Mundo
• Unchecked Power moves to trash
• Mundo gains +1 power passively (now at 13 power)
• Cleanup occurs when Unchecked leaves the chain
• Mundo has 13 power with 12 marked damage and survives

Nuances:

• Cleanups do not happen in the middle of card resolution
• The passive power gain from the card entering the graveyard occurs before the cleanup that checks for lethal damage