Sacrifice Eternal Witness to Phyrexian Altar, for {B}.
Cast Unearth, returning Eternal Witness from your graveyard to the battlefield.
Eternal Witness enters, returning Unearth from your graveyard to your hand.
Repeat.

Infinite death triggers

Infinite creature ETB

Infinite creature LTB

Infinite creature sacrifice triggers

Infinite storm count

Infinite magecraft triggers

Eternal Witness

Phyrexian Altar

Unearth

Cast Unearth by paying {B}, targeting any applicable target.
Holding priority, cast Dualcaster Mage by paying {1}{R}{R}. 
When Dualcaster Mage enters, it triggers, targeting Unearth.
Holding priority, activate Ashnod's Altar by sacrificing Dualcaster Mage.
Resolve the Dualcaster Mage trigger, copying Unearth, with the copy targeting Dualcaster Mage.
Resolve the Unearth copy, returning Dualcaster Mage from your graveyard to the battlefield.
Repeat from step 3.

Infinite colorless mana

Ashnod's Altar

Dualcaster Mage

Cast Spellseeker, stacking Inalla's trigger under Spellseeker's ETB trigger.
Tutor Culling the Weak to hand with Spellseeker's ability.
Cast Culling the Weak, sacrificing Spellseeker and adding {B}{B}{B}{B}.
Resolve and pay for the Inalla trigger from earlier, creating a token copy of Spellseeker and leaving {B}{B}{B} floating.
When the token ETBs, tutor Unearth to your hand.
Unearth Spellseeker, leaving {B}{B} floating.
Shortcut resolving both Inalla's trigger and Spellseeker's ETB, tutoring for Burnt Offering and Finale of Promise, leaving {B} floating.
Cast Burnt Offering, sacrificing Spellseeker and adding {B}{R}{R}.
Use that mana to cast Finale of Promise with X=1 targeting Unearth and Culling the Weak.
Sacrifice a Spellseeker token to Culling the Weak for {B}{B}{B}{B} and Unearth Spellseeker.
Resolve Spellseeker's ETB and Inalla's trigger, tutoring Entomb and Reanimate into hand.
Entomb and then Reanimate Archaeomancer, leaving {B} floating.
Resolve Archaeomancer's ETB, returning Burnt Offering from your graveyard to your hand.
Cast Burnt Offering on Archaeomancer, adding {B}{B}{B}{B}.
Resolve the Inalla trigger from when Archaeomancer ETB'd and return Reanimate to your hand, leaving {B}{B}{B} floating.
Reanimate Archaeomancer and resolve the Inalla trigger, retrieving Burnt Offering and FInale of promise, {B} floating.
Sacrifice Archaeomancer to Burnt Offering, adding {R}{R}{B}{B}.
Cast Finale of Promise with X=1 targeting Entomb and Reanimate.
Entomb Shallow Grave and Reanimate Archaeomancer, {B} floating.
Resolve the Archaeomancer trigger returning Burnt Offering from your graveyard to your hand.
Sacrifice Archaeomancer to Burnt Offering, adding {B}{B}{B}{B}.
Resolve the Inalla trigger, making a token copy of Archaeomancer with haste and leaving {B}{B}{B} floating.
Return Shallow Grave with the token copy's ETB.
Cast Shallow Grave targeting Archaeomancer, leaving {B} floating.
Repeat steps 20-24 for infinite hasty Archaeomancers.

Infinite creature tokens with haste

Inalla, Archmage Ritualist

Spellseeker

Culling the Weak

Shallow Grave

Burnt Offering

Finale of Promise

Reanimate

Entomb

Archaeomancer

Activate Phyrexian Altar by sacrificing Phantasmal Image, adding {B}.
When Phantasmal Image dies, it triggers, causing you to mill six cards and return Unearth and up to one other instant or sorcery card from your graveyard to your hand.
Cast Unearth by paying {B}, returning Phantasmal Image from your graveyard to the battlefield as a copy of Kairi.
Repeat.

Infinite self-mill

Return all instant and sorcery cards from your graveyard to your hand

Kairi, the Swirling Sky

Phantasmal Image

Sakashima of a Thousand Faces

Activate Phyrexian Altar by sacrificing Protean Raider, adding {B}.
When Protean Raider dies, it triggers, causing you to mill six cards and return Unearth and up to one other instant or sorcery card from your graveyard to your hand.
Cast Unearth by paying {B}, returning Protean Raider from your graveyard to the battlefield as a copy of Kairi.
Repeat.

Protean Raider

Council of Reeds

Combos with "Unearth"