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.

Creatures opponents control die upon entering

Lock

Reduce the toughness of creatures opponents control to 0

Maha, Its Feathers Night

M.O.D.O.K.

Activate M.O.D.O.K. by paying 3 life.
Vilis triggers, causing you to draw three cards.
Sheoldred's first ability triggers three times, causing you to gain 6 life.
Repeat as desired.
Resolve all M.O.D.O.K. abilities.

Infinite card draw

Infinite draw triggers

Near-infinite +1/+1 counters on a creature

Near-infinite draw triggers

Near-infinite lifegain

Near-infinite lifegain triggers

Near-infinite looting

Near-infinite self-discard triggers

Sheoldred, the Apocalypse

Vilis, Broker of Blood

Activate M.O.D.O.K. by paying 3 life.
Repeat step 1 until you have 0 life.
Resolve all M.O.D.O.K. abilities.
Cast Repay in Kind by paying {5}{B}{B}, causing each opponent's life total to become 0.
Each opponent loses the game as a state-based action due to having 0 life.

Near-infinite lifeloss for all players

Each opponent loses the game

Permanent with "You don't lose the game due to having 0 or less life"

Repay in Kind

Activate M.O.D.O.K. by paying life.
Repeat step 1 as desired.
Activate Rowan by tapping it, reducing the cost of all black and/or red spells you cast this turn.
Cast Exsanguinate, choosing for X to equal the amount of life you've lost this turn, but paying a total of {B}{B} due to Rowan.
Resolve Exsanguinate, causing each opponent to lose X life, and you to gain life equal to the life lost this way.

Near-infinite lifeloss

Rowan, Scion of War

Exsanguinate

Activate M.O.D.O.K. by paying life.
Repeat step 1 until you have 0 life.
Cast Mister Negative by paying {5}{W}{B} plus commander tax if applicable.
When Mister Negative enters, it triggers, swapping your life total with an opponent's.
The opponent loses the game due to having 0 life.

Near-infinite lifeloss for target opponent

Target opponent loses the game

Mister Negative

Phyrexian Unlife

Activate Vizkopa Guildmage's second ability by paying {1}{W}{B}, causing each opponent to lose life whenever you gain life this turn.
Activate M.O.D.O.K. by paying 3 life.
Repeat step 2 until you have 3 or less life.
Activate Children of Korils by sacrificing it, causing you to gain life equal to the amount of life you've lost this turn.
Vizkopa Guildmage triggers, causing each opponent to lose life equal to the amount of life you gained.

Children of Korlis

Vizkopa Guildmage

When a land enters the battlefield, it becomes a Swamp due to Kormus Bell. This then causes it to become a 0/0 creature due to Urborg and M.O.D.O.K., and die as a state-based action due to having 0 toughness.

Destroy all lands

Destroy all lands that enter the battlefield

Mass Land Denial

Kormus Bell

Urborg, Tomb of Yawgmoth

Activate M.O.D.O.K. by paying life.
Repeat step 1 as desired.
Activate Rowan by tapping it, reducing the cost of all black and/or red spells you cast this turn.
Cast a copy of Exsanguinate, choosing for X to equal the amount of life you've lost this turn, but paying a total of {B}{B} due to Rowan.
Resolve Exsanguinate, causing each opponent to lose X life, and you to gain life equal to the life lost this way.

Stensian Sanguinist // Exsanguinate

Activate M.O.D.O.K. by paying 3 life.
Repeat step 1 until Greven has power 21 or greater due to its first ability.
Deal combat damage to an opponent using Greven. The opponent loses the game due to being dealt 21 or more combat damage from your commander.

Combos with "M.O.D.O.K."