Cast Romana II either from your hand or from the command zone by paying {3}{W} plus commander tax, if applicable.
The Sixth Doctor triggers, creating a token copy of Romana II that isn't legendary.
During another turn, cast Adrix and Nev by paying {2}{G}{U}.
The Sixth Doctor triggers, creating a token copy of Adrix and Nev that isn't legendary.
Resolve the Adrix and Nev spell.
Activate a Romana II by paying {1} and tapping it, creating four token copies of the Adrix and Nev token that aren't legendary.
Activate the other Romana II by paying {1} and tapping it, creating 64 token copies of the Adrix and Nev token that aren't legendary.
Whenever you create a token from this point on, you will create 2^70 times as many of those tokens due to the 70 Adrix and Nev's you control.

Near-infinite ETB

Near-infinite creature tokens

Near-infinite token doubling

Romana II

The Sixth Doctor

Adrix and Nev, Twincasters

Cast Teferi by paying {2}{W}{U}.
The Sixth Doctor triggers, copying the Teferi spell, with the copy being nonlegendary.
Resolve both Teferi spells.
Activate a mana-producing land or artifact by tapping it, adding at least {1}.
Activate Romana II by paying {1} and tapping it, creating a copy of the Teferi token.
Activate a Teferi token's first loyalty ability by putting a loyalty counter on it, untapping Romana II, the mana-producing land or artifact and up to one other land or artifact, and causing you to gain 2 life.
Repeat from step 4.

Infinite lifegain triggers

Infinite lifegain

Infinite mana lands you control can produce

Infinite mana artifacts you control can produce

Infinite untap of lands you control

Infinite untap of artifacts you control

Teferi, Who Slows the Sunset

Cast Karn's Temporal Sundering by paying {4}{U}{U}.
The Sixth Doctor triggers, copying Karn's Temporal Sundering.
Resolve the copy of Karn's Temporal Sundering, causing you to take an extra turn after this one.
Holding priority, activate Isochron Scepter by paying {2} and tapping it, casting a copy of Remand without paying its mana cost.
Resolve Remand, returning the original Karn's Temporal Sundering to your hand and causing you to draw a card.
Repeat each turn.

Infinite turns

Karn's Temporal Sundering

Isochron Scepter

Remand

Cast Karn's Temporal Sundering by paying {4}{U}{U}.
The Sixth Doctor triggers, copying Karn's Temporal Sundering.
Resolve the copy of Karn's Temporal Sundering, causing you to take an extra turn after this one.
Holding priority, activate Isochron Scepter by paying {2} and tapping it, casting a copy of Reprieve without paying its mana cost.
Resolve Reprieve, returning the original Karn's Temporal Sundering to your hand and causing you to draw a card.
Repeat each turn.

Reprieve

Cast Karn's Temporal Sundering by paying {4}{U}{U}.
The Sixth Doctor triggers, copying Karn's Temporal Sundering.
Resolve the copy of Karn's Temporal Sundering, causing you to take an extra turn after this one.
Holding priority, activate Isochron Scepter by paying {2} and tapping it, casting a copy of Unsubstantiate without paying its mana cost.
Resolve Unsubstantiate, returning the original Karn's Temporal Sundering to your hand.
Repeat each turn.

Unsubstantiate

Cast Ugin's Nexus by paying {5}.
The Sixth Doctor triggers, copying the Ugin's Nexus spell, with the copy being nonlegendary.
Resolve both Ugin's Nexus spells.
Activate Vedalken Mastermind by paying {U} and tapping it, returning the nontoken Ugin's Nexus to your hand.
Activate Krark-Clan Ironworks by sacrificing the Ugin's Nexus token, adding {C}{C}.
The Ugin's Nexus token is exiled instead of being put into your graveyard, causing you to take an extra turn after this one.
Repeat each turn.

Ugin's Nexus

Vedalken Mastermind

Krark-Clan Ironworks

Cast Ugin's Nexus by paying {5}.
The Sixth Doctor triggers, copying the Ugin's Nexus spell, with the copy being nonlegendary.
Resolve both Ugin's Nexus spells.
Activate Krark-Clan Ironworks by sacrificing the Ugin's Nexus token, adding {C}{C}.
The Ugin's Nexus token is exiled instead of being put into your graveyard, causing you to take an extra turn after this one.
Activate Meticulous Excavation by paying {2}{W}, returning the nontoken Ugin's Nexus to your hand.
Repeat each turn.

Combos with "The Sixth Doctor"