The harder cards to justify playing are the less obvious answers
Number #184 Rarity rare Artist Daniel Gelon Sell this card
@PioneerDLs My take on Hardened Scales
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Achilles Davenport Adwal, Breaker of Chains Alela, Cunning Conqueror Alpharael, Dreaming Acolyte Anowon, the Ruin Thief Araumi of the Dead Tide Architects of Will Ashiok's Forerunner Atris, Oracle of Half-Truths Aven Heartstabber Azula, Cunning Usurper Baleful Strix Bane Alley Broker Basim Ibn Ishaq Benthic Djinn Bladestitched Skaab Blizzard Specter Callidus Assassin Captain N'ghathrod Carlo, Suave Schemer Cavern Harpy Cemetery Puca Circu, Dimir Lobotomist Consuming Aberration Contraband Kingpin Cunning Nightbonder Curious Cadaver Cybernetica Datasmith Cyclonus, the Saboteur Darkblade Agent Deadeye Brawler Deadeye Plunderers Deathcult Rogue Deceit Dee Kay, Finder of the Lost Dementia Sliver Devourer of Memory Dimir Cutpurse Dimir Doppelganger Dimir Guildmage Dimir Infiltrator Dimir Spybug Dimir Strandcatcher Dinrova Horror Diregraf Captain Disciple of Deceit Doc Ock, Evil Inventor Doctor Octopus, Master Planner Doomsday Specter Dragonlord Silumgar Dralnu, Lich Lord Dreadwing Scavenger Duskmantle Guildmage Duskmantle Seer Eloise, Nephalia Sleuth Emet-Selch of the Third Seat Emet-Selch, Unsundered Ertai Resurrected Etherium Abomination Etrata, Deadly Fugitive Etrata, the Silencer Evangel of Synthesis Evil Twin Ezio, Blade of Vengeance Faerie Snoop Fallen Shinobi Fathom Feeder Fear of Infinity Foggy Swamp Spirit Keeper Garland, Royal Kidnapper Ghastlord of Fugue Ghost, Spectral Saboteur

As long as it deals combat damage every turn, you can cast any spell you want without paying its mana cost