All creatures your opponents control will have base toughness 0, and die as a state-based action due to having 0 toughness unless there is another effect increasing its toughness.
Whenever a creature enters the battlefield under an opponent's control, it will have base toughness 0, and die as a state-based action due to having 0 toughness unless there is another effect increasing its toughness.

Reduce each opposing creature's toughness to 0

Creatures opponents control die upon entering the battlefield

Maha, Its Feathers Night

Kaervek, the Spiteful

At the beginning of your end step, Toxrill's first ability triggers, putting a slime counter on each creature your opponents control.
Because your opponents' creatures are now 0/0 due to Maha and Toxrill's static effects, the creatures die as a state-based action due to having zero toughness unless there is an effect boosting their toughness.
Repeat during each of your end steps.

Reduce the toughness of opponents' creatures to 0 during each of your end steps

Toxrill, the Corrosive

Night of Souls' Betrayal

During any turn, activate Pestilence by paying {B}, dealing 1 damage to each creature and each player.
Each creature your opponents control dies as a state-based action due to having 0 toughness.
Repeat each turn as desired.

Destroy all creatures opponents control during each turn

Pestilence

All creatures your opponents control will have base toughness -1, and die as a state-based action due to having 0 or less toughness unless there is another effect increasing its toughness.
Whenever a creature enters the battlefield under an opponent's control, it will have base toughness -1, and die as a state-based action due to having 0 or less toughness unless there is another effect increasing its toughness.

Elesh Norn, Grand Cenobite

During any turn, activate Pestilence Demon by paying {B}, dealing 1 damage to each creature and each player.
Each creature your opponents control dies as a state-based action due to having 0 toughness.
Repeat each turn as desired.

Pestilence Demon

Activate Goblin Sharpshooter by tapping it, dealing 1 damage to a creature an opponent controls.
The creature dealt damage dies as a state based action due to having 0 or less toughness, triggering Goblin Sharpshooter, untapping Goblin Sharpshooter.
Repeat as desired.
You may also repeat this loop whenever a creature enters the battlefield under an opponent's control.

Destroy all creatures that enter the battlefield under an opponent's control

Destroy all creatures opponents control

Goblin Sharpshooter

During any turn, activate Pyrohemia by paying {R}, dealing 1 damage to each creature and each player.
Each creature your opponents control dies as a state-based action due to having 0 toughness.
Repeat each turn as desired.

Pyrohemia

All nonwhite creatures your opponents control will have base toughness 0, and die as a state-based action due to having 0 toughness unless there is another effect increasing its toughness.
Whenever a nonwhite creature enters the battlefield under an opponent's control, it will have base toughness 0, and die as a state-based action due to having 0 toughness unless there is another effect increasing its toughness.

Combos with "Maha, Its Feathers Night"