Endling

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.

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

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 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 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.