Cast Phyrexian Metamorph by paying {3}{U/P}, having it enter the battlefield as a copy of Gyruda.
Put Phyrexian Metamorph into your graveyard due to the legend rule.
Mortuary triggers, putting Phyrexian Metamorph on top of your library from your graveyard.
Phyrexian Metamorph's enter the battlefield ability triggers, causing each player to mill four cards and you to return Phyrexian Metamorph from your graveyard to the battlefield as a copy of Gyruda.
Repeat from step 2.

Infinite death triggers

Infinite creature ETB

Infinite creature LTB

Infinite mill

Infinite creature sacrifice triggers

Infinite self-mill

Gyruda, Doom of Depths

Mortuary

Phyrexian Metamorph

Cast Clever Impersonator by paying {2}{U}{U}, having it enter the battlefield as a copy of Gyruda.
Put Clever Impersonator into your graveyard due to the legend rule.
Mortuary triggers, putting Clever Impersonator on top of your library from your graveyard.
Clever Impersonator's enter the battlefield ability triggers, causing each player to mill four cards and you to return Clever Impersonator from your graveyard to the battlefield as a copy of Gyruda.
Repeat from step 2.

Clever Impersonator

Cast Phantasmal Image by paying {1}{U}, having it enter the battlefield as a copy of Gyruda.
Put Phantasmal Image into your graveyard due to the legend rule.
Mortuary triggers, putting Phantasmal Image on top of your library from your graveyard.
Phantasmal Image's enter the battlefield ability triggers, causing each player to mill four cards and you to return Phantasmal Image from your graveyard to the battlefield as a copy of Gyruda.
Repeat from step 2.

Phantasmal Image

Cast Clone by paying {3}{U}, having it enter the battlefield as a copy of Gyruda.
Put Clone into your graveyard due to the legend rule.
Mortuary triggers, putting Clone on top of your library from your graveyard.
Clone's enter the battlefield ability triggers, causing each player to mill four cards and you to return Clone from your graveyard to the battlefield as a copy of Gyruda.
Repeat from step 2.

Clone

Cast Evil Twin by paying {2}{U}{B}, having it enter the battlefield as a copy of Gyruda.
Put Evil Twin into your graveyard due to the legend rule.
Mortuary triggers, putting Evil Twin on top of your library from your graveyard.
Evil Twin's enter the battlefield ability triggers, causing each player to mill four cards and you to return Evil Twin from your graveyard to the battlefield as a copy of Gyruda.
Repeat from step 2.

Evil Twin

Cast Gyruda by paying {4}{U/B}{U/B} and commander tax, if applicable.
Gyruda enters the battlefield, triggering its ability.
Holding priority, activate Phyrexian Altar by sacrificing Gyruda, adding one mana of any color.
Gyruda dies, triggering Mortuary, putting Gyruda on top of your library from your graveyard.
Resolve the Gyruda ability from step 2, causing each player to mill four cards and you to return Gyruda from your graveyard to the battlefield.
Repeat from step 2.

Infinite colored mana

Phyrexian Altar

Cast Gyruda by paying {4}{U/B}{U/B} and commander tax, if applicable.
Gyruda enters the battlefield, triggering its ability.
Holding priority, activate Ashnod's Altar by sacrificing Gyruda, adding {C}{C}.
Gyruda dies, triggering Mortuary, putting Gyruda on top of your library from your graveyard.
Resolve the Gyruda ability from step 2, causing each player to mill four cards and you to return Gyruda from your graveyard to the battlefield.
Repeat from step 2.

Search Results