Does a creature with undying and a +1/+1 counter still return if it also has a -1/-1 counter?

Undying reads: 'When this creature dies, if it had no +1/+1 counters on it, return it to the battlefield under its owner's control with a +1/+1 counter on it.' (CR 702.92a). The key check is whether the creature had a +1/+1 counter on it at the moment it died.

However, CR 121.3 establishes that +1/+1 counters and -1/-1 counters on the same permanent are removed in pairs as a state-based action until only one type (or neither) remains (CR 704.5q). This 'annihilation' happens continuously, so a creature carrying both counter types will have them cancel out before you ever evaluate undying.

So the outcome depends on how many of each counter the creature has:

• If it has exactly one +1/+1 and one -1/-1 counter, both are removed, leaving it with no +1/+1 counters — undying triggers.
• If it has two +1/+1 counters and one -1/-1 counter, after annihilation it still has one +1/+1 counter — undying does NOT trigger.

Concrete example: Your Strangleroot Geist (which has undying) gets hit by Kulrath Knight's -1/-1 counter effect and has one of each counter. State-based actions immediately remove both, leaving it with no +1/+1 counters. When it dies later, undying sees no +1/+1 counter and the ability triggers, returning it with a +1/+1 counter.

The bottom line: the -1/-1 counter itself doesn't suppress undying — it's the net presence of a +1/+1 counter after annihilation that matters. Always resolve the counter cancellation (CR 704.5q) first, then evaluate what counters remain.