Activate Osgir by tapping it, paying {5}, and exiling Ugin's Nexus from your graveyard, creating two token copies of Ugin's Nexus.
Put one of them into your graveyard due to the legend rule.
The token copy of Ugin's Nexus that was just put into your graveyard triggers, causing you to take an extra turn after this one.
Activate Osgir by paying {1} and sacrificing a token copy of Ugin's Nexus.
The sacrificed token copy of Ugin's Nexus triggers, causing you to take an extra turn after this one.
Osgir's ability resolves, giving target creature you control +2/+0 until end of turn.
Move to your next extra turn.
Activate Isochron Scepter by tapping it and paying {2}, copying Pull from Eternity.
Cast Pull from Eternity, returning Ugin's Nexus to your graveyard.
Move to your next extra turn.
Repeat.

Infinite turns

Lock

Osgir, the Reconstructor

Ugin's Nexus

Pull from Eternity

Isochron Scepter

Activate Osgir's second ability by paying {5}, tapping it and exiling Ugin's Nexus from your graveyard, creating two token copies of Ugin's Nexus.
Exile one of the Ugin's Nexus copies due to the legend rule, causing you to take an extra turn after this one.
Activate Osgir's first ability by paying {1} and sacrificing the remaining Ugin's Nexus token.
The Ugin's Nexus token is exiled instead of being put into your graveyard, causing you to take an extra turn after this one.
Resolve the Osgir ability, giving a creature you control +2/+0 until end of turn.
Move to your first extra turn.
If you do not control a Mirror of Fate token, activate Osgir's second ability by paying {5}, tapping it and exiling Mirror of Fate from your graveyard, creating two token copies of Mirror of Fate.
Activate a Mirror of Fate token by tapping and sacrificing it, choosing Ugin's Nexus, any other card in exile, Mirror of Fate and up to four other cards in exile, then exiling your library and putting the chosen cards on top in the aforementioned order.
Activate Codex Shredder's first ability by tapping it, causing you to mill Ugin's Nexus.
Move to your second extra turn.
Activate Codex Shredder's first ability by tapping it, causing you to mill Mirror of Fate.
Repeat.

Mirror of Fate

Codex Shredder

Activate Osgir's second ability by paying {6}, tapping it and exiling Gonti's Aether Heart from your graveyard, creating two token copies of Gonti's Aether Heart.
Put one of the Gonti's Aether Hearts into your graveyard due to the legend rule.
Both Gonti's Aether Hearts' enters-the-battlefield ability trigger twice, adding a total of eight {E}.
Activate the remaining Gonti's Aether Heart by paying eight {E} and exiling it, causing you to take an extra turn after this one.
Activate Isochron Scepter by paying {2} and tapping it, casting a copy of Pull from Eternity without paying its mana cost.
Resolve Pull from Eternity, putting Gonti's Aether Heart into your graveyard from exile.
Repeat each turn.

Gonti's Aether Heart

Activate Osgir by paying {5}, tapping it and exiling Ugin's Nexus from your graveyard, creating four token copies of Ugin's Nexus.
Put three of the Ugin's Nexus tokens into exile due to the legend rule and their replacement effects, causing you to take three extra turns after this one.
Activate Karn's first loyalty ability by putting a loyalty counter on it, choosing to do nothing.
Move to your next extra turn.
Repeat from step 3 one additional time.
Activate Karn's second loyalty ability by removing two loyalty counters from it, returning Ugin's Nexus from exile to your hand.
Activate Harnfel by discarding Ugin's Nexus.
Move to your last extra turn.
Repeat.

Combos with "Osgir, the Reconstructor"