What it does
Undying means that when this creature dies (goes to the graveyard from the battlefield), if it had no +1/+1 counters on it, you return it to the battlefield under your control with a +1/+1 counter on it. So it comes back bigger, but only once—after that it has a counter and won't return again unless that counter is removed.
The big nuance: undying only checks for +1/+1 counters at the moment it dies. If the creature already has a counter (from undying or anything else), it stays dead. People also forget undying triggers on any death, not just combat—sacrifice effects work too, which makes it great with sacrifice synergies.
In Commander, undying shines with counter-removal (like -1/-1 effects or proliferate manipulation), sacrifice loops, and blink-style value engines. It's also resilient against single-target removal, forcing opponents to deal with the creature twice.













