Discard a land card and pay the mana cost (and other costs) of the card with Retrace, and you can cast if from your graveyard
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Like most Magic: The Gathering cards, the value of Krark, the Thumbless depends on a few consistent factors: its rarity, the printing and finish (regular or foil), its physical condition, and the set it was printed in (Magic Online Promos)

Swab Goblin Rarity: Common TYPE: Creature SUBTYPE: Goblin Pirate CMC: 2 Mana cost: USD: $0.09 EUR: 0.13 TIX: 0.03 EDHREC RANK: 24682 (2/2) Swamp Rarity: Common TYPE: Basic Land SUBTYPE: Swamp CMC: 0 Mana cost: USD: $0.23 EUR: 0.12 TIX: 0.03 EDHREC RANK: - ( : Add .) Swamp Rarity: Common TYPE: Basic Land SUBTYPE: Swamp CMC: 0 Mana cost: USD: $0.15 EUR: 0.10 TIX: 0.03 EDHREC RANK: - ( : Add .) Swamp Rarity: Common TYPE: Basic Land SUBTYPE: Swamp CMC: 0 Mana cost: USD: $0.61 EUR: 0.80 TIX: 0.03 EDHREC RANK: - ( : Add .) Swamp Rarity: Common TYPE: Basic Land SUBTYPE: Swamp CMC: 0 Mana cost: USD: $0.34 EUR: 0.31 TIX: 0.03 EDHREC RANK: - ( : Add .) Swiftblade Vindicator Rarity: Rare TYPE: Creature SUBTYPE: Human Soldier CMC: 2 Mana cost: USD: $0.26 EUR: 0.15 TIX: 0.02 EDHREC RANK: 7527 (1/1) Double strike (This creature deals both first-strike and regular combat damage.) Vigilance (Attacking doesn't cause this creature to tap.) Trample (This creature can deal excess combat damage to the player or planeswalker it's attacking.) Swiftfoot Boots Rarity: Uncommon TYPE: Artifact SUBTYPE: Equipment CMC: 2 Mana cost: USD: $2.15 EUR: 2.50 TIX: 0.16 EDHREC RANK: 12 Equipped creature has hexproof and haste

You make a creature that can make Treasures, you draw cards, and then you finally get a creature that can copy another creature for a turn