Activate Gruul Turf by tapping it, adding {R}{G}.
Activate Silvanus's Invoker by paying {1}, untapping Gruul Turf and causing it to become an 8/8 Elemental with trample and haste in addition to its other types until end of turn.
Repeat.
Once you have infinite mana, you may tap and untap any other land(s) you control, and use the mana to activate Agatha of the Vile Cauldron's ability.

Infinite mana lands you control can produce

Infinite red mana

Infinite green mana

Infinite power and toughness for most creatures you control until end of turn

Agatha of the Vile Cauldron

Silvanus's Invoker

Gruul Turf

Activate Castle Garenbrig's second ability by paying {2}{G}{G} and tapping it, adding six {G} that can only be spent to cast creature spells or to activate abilities of creatures.
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.
Repeat.
Once you have infinite mana, you may tap and untap any other land(s) you control, and if you control lands that can produce {R} and {G}, use the mana to activate Agatha of the Vile Cauldron's ability.

Castle Garenbrig

Activate Itlimoc by tapping it, adding at least nine {G}.
Activate Silvanus's Invoker by paying {8}, causing Itlimoc to become a 8/8 Elemental with haste, and untapping Itlimoc.
Repeat.

Growing Rites of Itlimoc // Itlimoc, Cradle of the Sun

Activate Nykthos' second ability by paying {2} and tapping it, causing you to add eleven mana of any color with which you have devotion.
Activate Silvanus's Invoker by paying {8}, causing Nykthos to become an 8/8 Elemental with haste until end of turn, and untapping it.
Repeat.

Nykthos, Shrine to Nyx

Activate Gaea's Cradle by tapping it, adding at least nine {G}.
Activate Silvanus's Invoker by paying {8}, causing Gaea's Cradle to become a 8/8 Elemental with haste, and untapping Gaea's Cradle.
Repeat.

Gaea's Cradle

Activate Guildless Commons by tapping it, adding {C}{C}.
Activate Silvanus's Invoker by paying {1}, untapping Guildless Commons and causing it to become an 8/8 Elemental with trample and haste in addition to its other types until end of turn.
Repeat.
Once you have infinite mana, you may tap and untap any other land(s) you control, and if you control lands that can produce {R} and {G}, use the mana to activate Agatha of the Vile Cauldron's ability.

Infinite colorless mana

Guildless Commons

Activate Jungle Basin by tapping it, adding {C}{G}.
Activate Silvanus's Invoker by paying {1}, untapping Jungle Basin and causing it to become an 8/8 Elemental with trample and haste in addition to its other types until end of turn.
Repeat.
Once you have infinite mana, you may tap and untap any other land(s) you control, and if you control lands that can produce {R} and {G}, use the mana to activate Agatha of the Vile Cauldron's ability.

Jungle Basin

Activate Dormant Volcano by tapping it, adding {C}{R}.
Activate Silvanus's Invoker by paying {1}, untapping Dormant Volcano and causing it to become an 8/8 Elemental with trample and haste in addition to its other types until end of turn.
Repeat.
Once you have infinite mana, you may tap and untap any other land(s) you control, and if you control lands that can produce {R} and {G}, use the mana to activate Agatha of the Vile Cauldron's ability.

Dormant Volcano

Activate Serra's Sanctum by tapping it, adding at least nine {W}.
Activate Silvanus's Invoker by paying {8}, causing Serra's Sanctum to become a 8/8 Elemental with haste, and untapping Serra's Sanctum.
Repeat.

Infinite white mana

Serra's Sanctum

Activate City of Traitors by tapping it, adding {C}{C}.
Activate Silvanus's Invoker by paying {1}, untapping City of Traitors and causing it to become an 8/8 Elemental with trample and haste in addition to its other types until end of turn.
Repeat.
Once you have infinite mana, you may tap and untap any other land(s) you control, and if you control lands that can produce {R} and {G}, use the mana to activate Agatha of the Vile Cauldron's ability.

City of Traitors

Activate Vault of Catlacan's second ability by tapping it, adding at least nine {U}.
Activate Silvanus's Invoker by paying {8}, causing Vault of Catlacan to become a 8/8 Elemental with haste, and untapping Vault of Catlacan.
Repeat.

Combos with "Silvanus's Invoker"