Mikaeus, the Unhallowed + Devoted Druid

Mikaeus, the Unhallowed
Devoted Druid

Requires
Mikaeus, the Unhallowed must be on the battlefield. Devoted Druid must be on the battlefield.
Steps
  1. Activate Devoted Druid's second ability by putting a -1/-1 counter on it, untapping it.
  2. Repeat step 1 until Devoted Druid dies as a state-based action due to having zero toughness.
  3. Devoted Druid dies, triggering its undying ability, returning it from your graveyard to the battlefield with a +1/+1 counter on it.
  4. Repeat.
Results
Infinite death triggers. Infinite ETB. Infinite LTB.