Agatha of the Vile Cauldron + Silvanus's Invoker + Castle Garenbrig

Agatha of the Vile Cauldron

Silvanus's Invoker

Castle Garenbrig

How it works

1. Activate Castle Garenbrig's second ability by paying 2 and tapping it, adding six that can only be spent to cast creature spells or to activate abilities of creatures.
2. Activate Silvanus's Invoker by paying 1, untapping Castle Garenbrig and causing it to become an 8/8 Elemental with trample and haste in addition to its other types until end of turn.
3. Repeat.
4. Once you have infinite mana, you may tap and untap any other land(s) you control, and if you control lands that can produce and , use the mana to activate Agatha of the Vile Cauldron's ability.

This ingenious loop lets you squeeze infinite mana and infinite untaps out of a single land by leaning heavily on cost-reduction math. With Agatha of the Vile Cauldron boosting your creatures' activation discounts based on her power, the usually prohibitive eight-mana cost of Silvanus's Invoker drops down to a mere single mana. That tiny investment is easily funded by Castle Garenbrig's massive mana-producing ability, which also happens to satisfy the restriction on what kind of mana can activate the Invoker. As you cycle through this loop, Castle Garenbrig repeatedly untaps and transforms into an 8/8 Elemental, generating a surplus of green mana with every iteration that fuels the entire engine.