Agatha of the Vile Cauldron + Silvanus's Invoker + Keeper of Progenitus

Agatha of the Vile Cauldron

Silvanus's Invoker

Keeper of Progenitus

How it works

1. Activate a Mountain, Forest or Plains by tapping it, adding 1.
2. Keeper of Progenitus triggers, adding 1.
3. Activate Silvanus's Invoker by paying 1, untapping the land and causing it to become an 8/8 Elemental with trample and haste in addition to its other types until end of turn.
4. Repeat.

This loop relies on Agatha of the Vile Cauldron to drastically slash the activation cost of Silvanus's Invoker down to a mere single mana, turning what is normally a hefty investment into an engine piece. Meanwhile, Keeper of Progenitus acts as the crucial mana doubler, ensuring that every time you tap a qualifying land to pay for the reduced ability, the extra mana generated covers the cost completely with a mana left over. By continuously paying {1} to untap the land and trigger the Keeper, you can repeat the process infinitely to generate arbitrary amounts of mana and creature stats.