You'll put a time counter on it at the beginning of your upkeep, and once you have at least one you'll be able to Time Travel (or proliferate) them to either of your hearts' content!

Abstract Paintmage Academy Journeymage Aethermage's Touch Alabaster Mage Alexi, Zephyr Mage Amphin Pathmage Ana Battlemage Arc Mage Archmage Ascension Archmage Emeritus Archmage of Echoes Archmage of Runes Archmage's Charm Archmage's Newt Armageddon Armageddon Clock Armorer Guildmage Ascended Lawmage Auratouched Mage Aven Wind Mage Aziza, Mage Tower Captain Azorius Guildmage Azorius thermage Azure Mage Balmor, Battlemage Captain Bant Battlemage Barrin, Tolarian Archmage Battlemage's Bracers Bazaar Trademage Beamsplitter Mage Beastmaster's Magemark Beetleform Mage Black Mage's Rod Blaster Mage Blinding Mage Blue Mage's Cane Boros Guildmage Brimstone Mage Brushstroke Paintermage Call Damage Control Callous Bloodmage Cephalid Inkmage Cephalid Pathmage Civic Guildmage Clan Guildmage Clinquant Skymage Collateral Damage Combine Guildmage Conclave Guildmage Cornered by Black Mages Coruscation Mage Crafty Pathmage Crimson Mage Cult Guildmage Cunning Geysermage Cunning Sparkmage Damage Control Crew Davriel, Rogue Shadowmage Deal Damage Deeproot Pilgrimage Defacing Duskmage Delina, Wild Mage Dimir Guildmage Dirgur Focusmage Disruptive Pitmage Dour Port-Mage Dragon Mage Dualcaster Mage Duergar Hedge-Mage Dungeon of the Mad Mage Duskmantle Guildmage Eager Glyphmage

Pull rate from Rare Packs: 2.38% The estimated pull rate for Tapu Koko (Celestial Guardians) from a Rare Pack is approximately 2.38%, meaning you might need to open around 43 Rare Packs to obtain it
Cards numbered beyond the printed set total (150/149 and up) receive an entirely gold-foil border-and-illustration finish that became the modern Secret Rare standard
Brightglass Gearhulk A new cycle of allied color Gearhulks offers a lot of potential, but none of them stand out to me more than Selesnyas Brightglass Gearhulk