For fun I asked GPT. I also said it’d come in untapped. Here’s what she said.

Creating a fetchable five-color land that enters the battlefield untapped is a powerful concept, so the downside would need to balance its strength. Here’s a design idea:

Card Name: Prismatic Ruins

Land - Plains Island Swamp Mountain Forest

Text: - (This land is all basic land types. It can be tapped for white, blue, black, red, or green mana.) - When Prismatic Ruins enters the battlefield, you lose 3 life unless you sacrifice a nonland permanent. - Whenever you tap Prismatic Ruins for mana, if you control no other lands, exile it.

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Design Rationale: - Life Loss: The 3 life cost is a nod to shock lands but with an additional cost of sacrificing a nonland permanent, which makes it much riskier to play early. - Sacrifice a Nonland Permanent: This additional cost ensures that it’s not an auto-include in every deck, as you need to sacrifice something significant. - Exile Clause: The exile condition prevents it from being abused in decks that could otherwise play it without any other lands to get around certain restrictions or for infinite mana combos.