Activate Phyrexian Altar by sacrificing Boneyard Scourge, adding {1}.
Activate Phyrexian Altar by sacrificing another Dragon, adding {B}.
When the Dragon dies, Boneyard Scourge triggers, causing you to pay {1}{B} to return it from your graveyard to the battlefield.
When Boneyard Scourge enters the battlefield, Lathliss triggers, creating a 5/5 Dragon creature token.
Repeat.

Infinite death triggers

Infinite ETB

Infinite LTB

Infinite sacrifice triggers

Lathliss, Dragon Queen

Boneyard Scourge

Phyrexian Altar

Activate Phyrexian Altar by sacrificing Boneyard Scourge, adding {1}.
Activate Phyrexian Altar by sacrificing another Dragon, adding {B}.
When the Dragon dies, Boneyard Scourge triggers, causing you to pay {1}{B} to return it from your graveyard to the battlefield.
When Boneyard Scourge enters the battlefield, Molten Echoes triggers, creating a token copy of Boneyard Scourge with haste.
Repeat.

Molten Echoes

Activate Phyrexian Altar by sacrificing Boneyard Scourge, adding {1}.
Activate Phyrexian Altar by sacrificing another Dragon, adding {B}.
When the Dragon dies, Boneyard Scourge triggers, causing you to pay {1}{B} to return it from your graveyard to the battlefield.
When Boneyard Scourge enters the battlefield, Miirym triggers, creating a token copy of Boneyard Scourge.
Repeat.

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