Activate Witch Engine by tapping it, targeting any opponent.
Holding priority, activate Ashnod's Altar by sacrificing Witch Engine, adding {C}{C}.
Resolve the Witch Engine ability, adding {B}{B}{B}{B} and not giving control of Witch Engine to an opponent due to it no longer being on the battlefield.
Cast Corpse Dance with buyback by paying {4}{B}, returning Witch Engine from your graveyard to the battlefield with haste, and returning Corpse Dance to your hand.
Repeat.

Infinite black mana

Infinite colorless mana

Infinite death triggers

Infinite ETB

Infinite LTB

Infinite magecraft triggers

Infinite sacrifice triggers

Infinite storm count

Witch Engine

Corpse Dance

Ashnod's Altar

Activate Witch Engine by tapping it.
Holding priority, cast Corpse Dance with buyback by paying {2}{B} due to Blood Funnel.
Blood Funnel triggers, causing you to sacrifice Witch Engine.
Resolve Corpse Dance, returning Witch Engine from your graveyard to the battlefield, giving it haste, and returning Corpse Dance to your hand.
Resolve the Witch Engine ability from step 1, adding {B}{B}{B}{B} and failing to give an opponent control of Witch Engine as it is a new object.
Repeat.

Blood Funnel

Activate Altar of Dementia by sacrificing Witch Engine, causing target player to mill cards equal to Witch Engine's power.
Activate Witch Engine's unearth ability by paying {2}{B}, returning it from your graveyard to the battlefield, giving it haste and exiling it at end of turn or when it leaves the battlefield.
Activate Witch Engine by tapping it.
Holding priority, activate Altar of Dementia by sacrificing Witch Engine.
Both Witch Engine's undying ability and Void Maw attempt to exile Witch Engine when it would be put into your graveyard.
Choose to apply the Void Maw replacement effect, exiling Witch Engine.
Resolve the Witch Engine ability from step 3, adding {B}{B}{B}{B}.
Resolve the Altar of Dementia ability, causing target player to mill cards equal to Witch Engine's power.
Activate Void Maw by putting Witch Engine from exile into your graveyard, giving Void Maw +2/+2 until end of turn.
Repeat from step 2.

Infinitely large Void Maw until end of turn

Infinite mill

Infinite self-mill

Sedris, the Traitor King

Void Maw

Altar of Dementia

Activate Khârn using Witch Engine's ability by tapping it, adding {B}{B}{B}{B} and causing an opponent to gain control of Khârn.
Khârn triggers, causing you to draw two cards.
Activate Captivating Crew by paying {4}, causing you to gain control of Khârn.
Khârn triggers, causing the opponent who controlled it to draw two cards.
Repeat.

Combos with "Witch Engine"