Properties
Wondrous item, rare. The key opens any mundane lock while both lock and key are submerged. Once per dawn as an action, the bearer creates a 20-foot-radius sphere of calm water centered on the key for 10 minutes. Natural and magical currents cannot enter, and creatures inside have advantage on checks to swim, repair, or operate submerged mechanisms.
Lore
Hale engineers cut the key from pressure glass and trapped one full tide inside it. The Curator kept it beside the master index so changing water could never disturb the official record.
Game Master guidance
The calm sphere is not breathable air and does not immobilize creatures or projectiles. It rewards preparation around currents while leaving pressure, darkness, enemies, and limited breath meaningful. A visible silver tide indicates whether the daily use remains.
Related use
The item is designed as a memorable reward rather than a requirement for later play. A party without it can solve every related scene with the listed checks, tools, spells, or environmental choices.