Letalmorfa

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 Endling's power is reduced to 0 or less while in combat and it gains deathtouch, it won't deal damage, so deathtouch won't apply.

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

If a creature has +1/+1 counters and -1/-1 counters on it, state-based actions remove the same number of each so that it has only one kind of those counters on it. Endling's undying ability can bring it back again if its +1/+1 counters are removed this way.

If a creature has multiple instances of undying, they'll each trigger separately, but once one of those abilities returns the card to the battlefield, any others will have no effect. The creature won't receive multiple +1/+1 counters.

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 you activate Endling's last ability, you don't choose whether Endling gets +1/-1 or -1/+1 until that ability resolves.

Multiple instances of menace or deathtouch on the same creature are redundant.

Once undying returns the creature, it's considered a new object with no relation to the object that it was. Auras that were attached to it won't return to the battlefield. Equipment that was attached to it will remain unattached. Any counters that were on it won't be put on the new creature.