It alone turns the Cat/Oven combo into a machine gun, and its the card youll definitely not want to see in a sacrifice mirror match since it turns your game plan back at you
You have no max hand size and then start your engines and then when you've got max speed you draw two cards instead of one
According to Thursday's Dominaria United preview stream, they were discovered in long-forgotten cases in "a warehouse deep in the middle of nowhere, in Washington state"

Archway of Innovation Arena of Glory Argoth, Sanctum of Nature Ba Sing Se Barad-dr Blackcleave Cliffs Blooming Marsh Botanical Sanctum Canopy Vista Castle Ardenvale Castle Embereth Castle Garenbrig Castle Locthwain Castle Vantress Cinder Glade Clifftop Retreat Concealed Courtyard Copperline Gorge Coral Atoll Darkslick Shores Deathcap Glade Deserted Beach Dormant Volcano Dragonskull Summit Dreamroot Cascade Drowned Catacomb Dwarven Mine Everglades Fortified Beachhead Gingerbread Cabin Glacial Fortress Hall of Storm Giants Haunted Ridge Hinterland Harbor Idyllic Grange Inspiring Vantage Isolated Chapel Jungle Basin Karoo Minas Tirith Mines of Moria Monumental Henge Mystic Sanctuary Overgrown Farmland Prairie Stream Public Thoroughfare Razorverge Thicket Rivendell Rockfall Vale Rootbound Crag Seachrome Coast Shattered Sanctum Shifting Woodland Shipwreck Marsh Smoldering Marsh Spirebluff Canal Spymaster's Vault Stormcarved Coast Sulfur Falls Sundown Pass Sunken Hollow Sunpetal Grove Temple of the Dragon Queen The Shire Thran Portal Witch's Cottage Woodland Cemetery

While their values will shift depending on how much play they each see in Standard, it's safe to bet that these will hold up value-wise better than many of the aforementioned mythic rares
Still, there are situations where youve already put Griselbrand into play and need to kill them that turn