That's no cube, it's a Battlebox! One that will change with every game. It begins as only common cards, but each game the winner will be allowed to switch out one card for a new one.
Normal Battlebox rules (1 communal deck, land sets per player, 4 card opening hand, no mulligans, etc.)
When a player wins the game, they may exchange one card they "own" for a new card of their choice, with the following restrictions:
Agatha: Sure Strike -> Arc Blade
Ariel: Shaman of Spring -> Azusa's Many Journeys
Ariel: Gobhobbler Rats -> Fire Covenant
Ariel: Terrus Wurm -> Heirloom Mirror
Agatha: Greater Basilisk -> Become Immense
Agatha: Snakeform -> Vilespawn Spider
Agatha: Netcaster Spider -> Infuse with the Elements
Agatha: Goblin Test Pilot -> Emberwilde Caliph
Agatha: Riot Spikes -> Merciless Javelineer
Ariel: Lava Axe -> Magmatic Sinkhole
Ariel: Deadbridge Shaman -> Agonizing Remorse
Ariel: Regal Leosaur -> Tiana, Ship's Caretaker
Ariel: Scroll Thief -> Enclave Cryptologist
Ariel: Hoof Skulkin -> Circuit Mender
Mia: Lim-Dul's Paladin -> Labyrinth Raptor
Ariel: Battlewing Mystic -> Disallow