Hancock dies, its undying ability triggers, returning it from your graveyard to the battlefield with a +1/+1 counter on it.
Resolve the
Yahenni ability.
Activate
Yahenni by sacrificing
Hancock.
When
Hancock dies, its persist ability triggers, returning it from your graveyard to the battlefield with a -1/-1 counter on it.
Hancock enters and dies as a state-based action due to having 0 toughness. Its undying ability triggers, returning it from your graveyard to the battlefield with a +1/+1 counter on it.
Resolve the
Yahenni ability from step 4.
Repeat from step 4.
Notes
If
Hancock starts with a +1/+1 counter on it, then you may start the combo from step 4 instead.
Results
Infinite creature ETB.
Infinite creature LTB.
Infinite death triggers.
Infinite creature sacrifice triggers.
Metadata
Bracket tag
ExhibitionBracket 1+
For any deck
1
2
3
4
5
1 · Exhibition5 · cEDH
Bracket tags are estimated from the cards and results of each combo, as a guideline rather than a strict classification.