You control at least two other creatures with toughness equal to 1.
You have a way to put a -1/-1 counter on a creature.
Steps
Put a -1/-1 counter on any creature you control with toughness equal to 1. The creatue dies as a state-based action due to having zero toughness, triggering
Hapatra and
Host of the Hereafter.
Resolve the
Hapatra trigger, creating a 1/1 Snake creature token.
Resolve the
Host of the Hereafter trigger, putting a -1/-1 counter on any creature you control with toughness equal to 1.
Repeat from step 2.
Notes
You can alternatively start the combo with one creature that has toughness equal to 1 and another creature that has a -1/-1 counter on it. If you do, kill the creature with a -1/-1 counter on it to start the combo.