How can you create a combo with Yahenni, Undying Partisan, Agatha's Soul Cauldron, Walking Ballista?
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[START OF COMBO]This combo can be formed with Yahenni, Undying Partisan, Agatha's Soul Cauldron, Walking Ballista

Color identity: B
Mana cost: 
Steps:
  1. Give Yahenni deathtouch, if needed.
  2. Activate Yahenni using Walking Ballista's second ability by removing a +1/+1 counter from it, dealing 1 damage to a creature an opponent controls, and destroying it due to Yahenni's deathtouch.
  3. Yahenni triggers, putting a +1/+1 counter on it.
  4. Repeat from step 2 as desired.
  5. If you can give Yahenni deathtouch indefinitely, you may repeat this loop whenever a creature enters the battlefield under an opponent's control.[END OF COMBO STEPS]

Result:
  - Destroy all creatures opponents control
  - Destroy any creature that enters the battlefield under an opponent's control[END OF COMBO RESULT][END OF COMBO]

Other prerequisites:
  - Yahenni, Undying Partisan must be on the battlefield
  - Agatha's Soul Cauldron must be on the battlefield
  - Walking Ballista must be exiled
  - Yahenni has at least one +1/+1 counter on it.
  - You have a way to give Yahenni deathtouch.
[END OF COMBO PREREQUISITES]
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