mtg cards like heartstone — Rulings, Legalities & Similar This effect cant reduce the mana in that cost to less than one mana. Combo Heartstone +Basalt Monolith +Karn,

Similarly, its important to keep yourself protected while jumping into the fray: Magus of the Moat will likely lock most opponents out of combat, and Ghostly Prison will disincentivize attacks in your direction

You put scry 3 on a Doom Blade and you have something massively over powered

Ambush Viper

Gush (T1, Ext, S, Block) Gush fits into a strange niche

Its also one of the more mentally taxing mechanics for paper tournament play, since a revealed Companion puts the onus on both players to regularly verify that the condition is not violated (something that seems likely to become an accidental issue post-sideboarding in draft)