Caged Sun is a mythic Artifact card from The Brothers' War Retro Artifacts (BRR), collector number 9. Current market price: $3.90 (foil $5.13), TCGplayer market prices via Scryfall.
As this artifact enters, choose a color. Creatures you control of the chosen color get +1/+1. Whenever a land's ability causes you to add one or more mana of the chosen color, add an additional one mana of that color.