Magosi, the Waterveil + Nesting Grounds + Contagion Engine

Prerequisites

Magosi has at least one eon counter on it.

Steps

1. Activate Contagion Engine by paying {4} and tapping it, putting an eon counter on Magosi.
2. Activate Nesting Grounds's last ability by paying {1} and tapping it, moving an eon counter from Magosi to Nesting Grounds.
3. 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.
4. Play Magosi from your hand.
5. Move to your next turn.
6. Activate Contagion Engine by paying {4} and tapping it, putting an eon counter on Nesting Grounds.
7. Activate Nesting Grounds's last ability by paying {1} and tapping it, moving an eon counter from Nesting Grounds to Magosi.
8. Repeat from step 3.