Great Hall of the Biblioplex is a rare Land card from Secrets of Strixhaven Promos (PSOS), collector number 257p. Current market price: $1.06 (foil $2.23), TCGplayer market prices via Scryfall.
{T}: Add {C}. {T}, Pay 1 life: Add one mana of any color. Spend this mana only to cast an instant or sorcery spell. {5}: If this land isn't a creature, it becomes a 2/4 Wizard creature with "Whenever you cast an instant or sorcery spell, this creature gets +1/+0 until end of turn." It's still a land.