Endling

If a creature with undying that has +1/+1 counters on it receives enough -1/-1 counters to cause it to be destroyed by lethal damage or put into its owner's graveyard for having 0 or less toughness, undying won't trigger and the card won't return to the battlefield. That's because undying checks the creature as it last existed on the battlefield, and it still had +1/+1 counters on it at that point.

If Endling's last ability gives it -1/+1 enough times, it may have negative power. Use that negative value if its power is further modified. For example, if Endling's last ability gives it -1/+1 five times, it's -2/8. Giving it +1/-1 twice after that makes it 0/6, not 2/6.

If a card leaves the graveyard after it dies but before the undying trigger resolves, it won't be returned to the battlefield.