Thelonite MonkHow it works & interactions with other cards

Natural Terraforming

The core ability of Thelonite Monk allows you to tap the creature and sacrifice a green creature as a cost. Once activated, you target a land, and that land becomes a Forest. This change is indefinite, meaning it does not wear off at the end of the turn and remains in effect as long as the land stays on the battlefield.

Subtypes and Supertypes

When a land becomes a Forest, it loses all other land types and any previously held mana abilities, but it gains the ability to tap for green mana. Importantly, as per official rulings, this effect does not add or remove the 'snow' supertype. If you target a Snow-Covered Mountain, it will become a Snow-Covered Forest.

Strategic Sacrifices

The cost of this ability is specific: you must sacrifice a green creature. You cannot sacrifice Thelonite Monk itself unless it is currently green (which it is, by default). Common mistakes include trying to target a land that is already a Forest; while legal, it generally has no effect unless you are trying to remove other land types from a non-basic land.

Interaction examples

Recycling the Witness

Eternal Witness

After you play Eternal Witness and return a card from your graveyard to your hand, its primary job is done. You can then tap Thelonite Monk and sacrifice Eternal Witness (a green creature) to turn an opponent's troublesome non-basic land into a basic Forest, effectively cutting off their access to specific colors or utility powers.

The Colorless Limitation

Solemn Simulacrum

If you have Solemn Simulacrum on the battlefield, you might want to sacrifice it to draw a card. However, Thelonite Monk's ability requires the sacrifice of a green creature. Since Solemn Simulacrum is a colorless artifact creature, it cannot be used as the cost for Thelonite Monk's ability unless another effect has changed its color to green.