Hardened Scales adds more +1/+1 counters to friendly creatures when any such counters are put on them, which adds up quickly
The first time split cards were seen was all the way back in 2000 during the Invasion Block, where each spell of the two spells would be a single colour

Magic: The Gathering Alpha Rare (5) Out of Stock Out of Stock Magic: The Gathering Kaladesh Rare (96) From: $4.99 Out of Stock 3 Foil Near Mint $ 4.99 Magic: The Gathering Kaldheim Rare (20) From: $6.99 Out of Stock 1 Played $ 6.99 Magic: The Gathering Modern Horizons 2 Common (18) From: $0.35 20+ Near Mint $ 0.35 20+ Foil Near Mint $ 0.75 1 Foil Played $ 0.65 Magic: The Gathering Innistrad: Crimson Vow Rare (11) From: $8.99 9 Near Mint $ 8.99 2 Foil Near Mint $ 10.99 Magic: The Gathering Outlaws of Thunder Junction Common (8) From: $0.29 20+ Near Mint $ 0.29 20+ Foil Near Mint $ 0.49 Magic: The Gathering Shadows over Innistrad Uncommon (17) From: $0.20 20+ Near Mint $ 0.29 2 Played $ 0.20 1 Foil Near Mint $ 1.25 Magic: The Gathering Lorwyn Rare (41) From: $8.99 4 Near Mint $ 9.99 1 Played $ 8.99 1 Foil Near Mint $ 27.99 Magic: The Gathering Return to Ravnica Common (52) From: $0.20 20+ Near Mint $ 0.29 6 Played $ 0.20 1 Foil Near Mint $ 0.99 Magic: The Gathering Ravnica City of Guilds Common (76) From: $0.49 11 Near Mint $ 0.59 5 Played $ 0.49 2 Foil Near Mint $ 10.99 Magic: The Gathering Dark Ascension Rare (112) From: $0.49 20+ Near Mint $ 0.49 Magic: The Gathering Bloomburrow: Commander Common (2) From: $0.49 20+ Near Mint $ 0.49 Magic: The Gathering Dominaria United Common (15) From: $0.35 20+ Near Mint $ 0.35 20+ Foil Near Mint $ 1.49 Magic: The Gathering Tarkir: Dragonstorm Commander Common (3) From: $0.75 16 Near Mint $ 0.75 Magic: The Gathering Secrets of Strixhaven Commander Common (3) From: $0.49 20 Near Mint $ 0.49 Magic: The Gathering Oath of the Gatewatch Rare (80) From: $0.85 19 Near Mint $ 0.99 1 Played $ 0.85 Magic: The Gathering Khans of Tarkir Common (50) From: $0.20 20+ Near Mint $ 0.29 2 Played $ 0.20 9 Foil Near Mint $ 0.75 Magic: The Gathering Apocalypse Rare (34) Out of Stock Out of Stock Magic: The Gathering Strixhaven: School of Mages Rare (20) From: $1.25 1 Near Mint $ 1.25 2 Foil Near Mint $ 1.75 Magic: The Gathering Modern Horizons Rare (28) From: $7.99 5 Near Mint $ 7.99 Magic: The Gathering Ravnica City of Guilds Rare (55) From: $17.99 Out of Stock 6 Played $ 17.99 1 Foil Played $ 98.99 Magic: The Gathering Tempest Rare (8) From: $2.49 19 Near Mint $ 2.99 8 Played $ 2.49 Magic: The Gathering Wilds of Eldraine Rare (8) From: $1.99 17 Near Mint $ 1.99 20+ Foil Near Mint $ 2.49 Magic: The Gathering Modern Horizons 3 Uncommon (17) From: $1.99 18 Near Mint $ 1.99 2 Foil Near Mint $ 3.49 Magic: The Gathering Kaldheim Uncommon (6) From: $0.20 20+ Near Mint $ 0.29 5 Played $ 0.20 6 Foil Near Mint $ 0.59
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Since the mana generated can only be used for Artifacts, this card was restricted instead of banned
There are several ways to use them, including minigames and contests, but none of those things are possible if you don't acquire them first, and acquiring monsters is what Carnival of Monsters is about
We can even start it with Mansherry