Cast the creature with MV<=2 by paying its mana cost if it doesn't have {X} in its mana cost, or by paying enough mana where X=1 otherwise, causing the creature to enter the battlefield with a +1/+1 counter on it.
Activate Ashnod's Altar by sacrificing the creature, adding {C}{C}.
When the creature dies, Reyhan triggers, putting a +1/+1 counter on Evolution Witness.
Evolution Witness triggers, returning the creature from your graveyard to your hand.
Repeat.
If the creature can be cast for {C} or less, this combo nets infinite colorless mana, and you may cast that creature for an arbitrarily large amount of mana if it has {X} in its mana cost.

Infinite +1/+1 counters on a creature

Infinite colorless mana

Infinite death triggers

Infinite creature ETB

Infinite creature LTB

Infinite creature sacrifice triggers

Infinite storm count

Creature with MV<=2 that is colorless and enters the battlefield with a +1/+1 counter on it

Evolution Witness

Ashnod's Altar

Reyhan, Last of the Abzan

Cast the creature from your hand by paying {2} or less, causing the creature to enter the battlefield with a +1/+1 counter on it.
Activate Ashnod's Altar by sacrificing the creature, adding {C}{C}.
When the creature dies, Host of the Hereafter triggers, putting a +1/+1 counter on Evolution Witness.
Evolution Witness triggers, returning the creature from your graveyard to your hand.
Repeat.
If the creature can be cast for {C} or less, this combo nets infinite colorless mana, and you may cast that creature for an arbitrarily large amount of mana if it has {X} in its mana cost.

Host of the Hereafter

Cast the creature from your hand by paying {2} or less, causing the creature to enter the battlefield with a +1/+1 counter on it.
Activate Ashnod's Altar by sacrificing the creature, adding {C}{C}.
When the creature dies, Resourceful Defense triggers, putting a +1/+1 counter on Evolution Witness.
Evolution Witness triggers, returning the creature from your graveyard to your hand.
Repeat.
If the creature can be cast for {C} or less, this combo nets infinite colorless mana, and you may cast that creature for an arbitrarily large amount of mana if it has {X} in its mana cost.

Resourceful Defense

Cast the creature from your hand by paying {2} or less, causing the creature to enter the battlefield with a +1/+1 counter on it.
Activate Ashnod's Altar by sacrificing the creature, adding {C}{C}.
When the creature dies, Reluctant Role Model triggers, putting a +1/+1 counter on Evolution Witness.
Evolution Witness triggers, returning the creature from your graveyard to your hand.
Repeat.
If the creature can be cast for {C} or less, this combo nets infinite colorless mana, and you may cast that creature for an arbitrarily large amount of mana if it has {X} in its mana cost.

Reluctant Role Model

Cast the creature from your hand by paying {2} or less, causing the creature to enter the battlefield with a +1/+1 counter on it.
Activate Ashnod's Altar by sacrificing the creature, adding {C}{C}.
When the creature dies, Yuna triggers, putting a +1/+1 counter on Evolution Witness.
Evolution Witness triggers, returning the creature from your graveyard to your hand.
Repeat.
If the creature can be cast for {C} or less, this combo nets infinite colorless mana, and you may cast that creature for an arbitrarily large amount of mana if it has {X} in its mana cost.

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