Crystal Shard

Most bounce is a one-shot: Unsummon resolves and is gone. The activated ability here is a permanent fixture, a repeatable engine that turns any creature with a useful enters-the-battlefield trigger into a value loop, re-buying the entrance each turn for the cost of re-casting the creature. The "unless its controller pays " clause is a soft tax rather than a hard lock: pointed across the table, it is a tempo nuisance, since the controller can simply pay the to keep the creature on the battlefield. The dual activation cost is the quiet flexibility. The mode lets blue-aligned shells fire the ability while keeping colorless mana free elsewhere; the all-colorless mode keeps it usable in any deck that just wants a recurring bounce, whatever its colors. The effect is identical either way, with only the activation price differing between the two options. That split, a cheap permanent that buys a slow, taxed, repeatable return-to-hand, made it a fixture of value-engine and combo toolkits long before "ETB loop" was common phrasing. It rewards creatures whose arrival matters more than their staying power, and it pays for that recursion honestly: every loop costs the creature's mana plus the activation, a price slow enough that an opponent's spare is usually all it takes to shrug off the bounce pointed their way.