Osgir, the Reconstructor + Ugin's Nexus + Mirror of Fate + Codex Shredder

Osgir, the Reconstructor

Ugin's Nexus

Mirror of Fate

Codex Shredder

How it works

1. 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.
2. Exile one of the Ugin's Nexus copies due to the legend rule, causing you to take an extra turn after this one.
3. Activate Osgir's first ability by paying 1 and sacrificing the remaining Ugin's Nexus token.
4. The Ugin's Nexus token is exiled instead of being put into your graveyard, causing you to take an extra turn after this one.
5. Resolve the Osgir ability, giving a creature you control +2/+0 until end of turn.
6. Move to your first extra turn.
7. 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.
8. 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.
9. Activate Codex Shredder's first ability by tapping it, causing you to mill Ugin's Nexus.
10. Move to your second extra turn.
11. Activate Codex Shredder's first ability by tapping it, causing you to mill Mirror of Fate.
12. Repeat.