Activate Karn's Bastion's last ability by paying {4} and tapping it, putting an eon counter on Magosi.
Activate Nesting Grounds's last ability by paying {1} and tapping it, moving an eon counter from Magosi to Nesting Grounds.
Activate Magosi's last ability by tapping it, removing an eon counter from it, and returning it to your hand, taking an extra turn after this one.
Play Magosi from your hand.
Move to your next turn.
Activate Karn's Bastion's last ability by paying {4} and tapping it, putting an eon counter on Nesting Grounds.
Activate Nesting Grounds's last ability by paying {1} and tapping it, moving an eon counter from Nesting Grounds to Magosi.
Repeat from step 3.

Infinite turns

Magosi, the Waterveil

Nesting Grounds

Karn's Bastion

Activate Karn's Bastion's second ability by paying {4} and tapping it.
Holding priority, activate Lithoform Engine's first ability by paying {2} and tapping it, copying Karn's Bastion's ability.
Resolve both Karn's Bastion abilities, putting four charge counters on Magistrate's Scepter.
Activate Magistrate's Scepter's second ability by tapping it and removing three charge counters from it, causing you to take an extra turn after this one.
Move to your next turn.
Activate Karn's Bastion's second ability by paying {4} and tapping it, putting two charge counters on Magistrate's Scepter.
Repeat steps 4 and 5 one time.
Repeat from step 1.

Tekuthal, Inquiry Dominus

Magistrate's Scepter

Lithoform Engine

Activate Karn's Bastion's second ability by paying {4} and tapping it, putting a charge counter on Magistrate's Scepter.
Activate Magistrate's Scepter's second ability by tapping it and removing three charge counters from it.
Kurkesh triggers, causing you to pay {R} to copy Magistrate's Scepter's ability.
Resolve both Magistrate Scepter abilities, causing you to take two extra turns after this one.
Move to your next turn.
Activate Magistrate's Scepter's first ability by paying {4} and tapping it.
Kurkesh triggers, causing you to pay {R} to copy Magistrate's Scepter's ability.
Resolve both Magistrate Scepter abilities, putting two charge counters on it.
Move to your next turn.
Repeat.

Kurkesh, Onakke Ancient

Doubling Season

Vorinclex, Monstrous Raider

Activate Karn's Bastion by paying {4}, putting a charge counter on Lost Jitte and two +1/+1 counters on Cryptic Trilobite.
Activate Cryptic Trilobite's first ability twice by removing two +1/+1 counters from it, adding {C}{C}{C}{C} that can only be spent to activate abilities.
Activate Lost Jitte by removing a charge counter from it, untapping Karn's Bastion.
Repeat.

Infinite proliferate

Lost Jitte

Cryptic Trilobite

Hardened Scales

Activate Karn's Bastion by paying {4}, putting a charge counter on Lost Jitte and Gemstone Array, and a +1/+1 counter on Cryptic Trilobite and Crystalline Crawler.
Activate Cryptic Trilobite's first ability by removing a +1/+1 counter from it, adding {C}{C} that can only be spent to activate abilities.
Activate Crystalline Crawler's first ability by removing a +1/+1 counter from it, adding {1}.
Activate Gemstone Array's second ability by removing a charge counter from it, adding {1}.
Activate Lost Jitte by removing a charge counter from it, untapping Karn's Bastion.
Repeat.

Crystalline Crawler

Gemstone Array

Activate Memnarch's first ability by paying {1}{U}{U}, causing Medomai the Ageless to become an artifact.
Deal combat damage to an opponent using Medomai the Ageless.
Medomai the Ageless triggers, causing you to take an extra turn after this one.
Cast Idris by paying {1}{U}{R}.
When Idris enters the battlefield, it triggers, exiling Medomai the Ageless until Idris leaves the battlefield.
Move to your extra turn.
At the beginning of your upkeep, Idris's vanishing ability triggers, removing a time counter from it.
Activate Karn's Bastion's second ability by paying {4} and tapping it, putting a time counter on Idris.
Deal combat damage to an opponent using Idris.
Idris triggers, causing you to take an extra turn after this one.
Repeat from step 6.

Combos with "Karn's Bastion"