Magosi, the Waterveil has at least two eon counters on it.
Magosi is untapped.
Steps
Activate
Nesting Grounds's last ability by paying 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 from the battlefield to your hand, taking an extra turn after this one.
Play Magosi.
At the beginning of your end step,
Atraxa triggers, causing you to proliferate, putting an additional eon counter on
Nesting Grounds.
On your next extra turn, activate
Nesting Grounds's last ability by paying and tapping it, moving an eon counter from
Nesting Grounds to Magosi.
Repeat from step 2.
Results
Infinite turns.
Metadata
Lock
Bracket tag
RuthlessBracket 4+
For competitive decks at brackets 4+
1
2
3
4
5
1 · Exhibition5 · cEDH
Bracket tags are estimated from the cards and results of each combo, as a guideline rather than a strict classification.