Yawgmoth by paying 1 life and sacrificing a creature, targeting any creature an opponent controls, putting a -1/-1 counter on a creature an opponent controls, and causing you to draw a card.
Hapatra and
Obelisk Spider's second abilities trigger.
Resolve the
Hapatra trigger, creating a 1/1 Snake creature token.
Resolve the
Obelisk Spider trigger, causing you to gain 1 life and each opponent to lose 1 life.
Repeat.
Notes
You may also repeat this loop whenever a creature enters the battlefield under an opponent's control.
Results
Near-infinite lifeloss.
Near-infinite lifegain triggers.
Near-infinite ETB.
Near-infinite LTB.
Near-infinite death triggers.
Near-infinite sacrifice triggers.
Near-infinite card draw.
Near-infinite draw triggers.
Destroy all creatures opponents control.
Destroy any creature that enters the battlefield under an opponent's control.