Source: Tales from the Yawning Portal

Wondrous item, uncommon

This whistle is carved from transparent crystal, and it resembles a tiny dragon curled up like a snail. The name Night Caller is etched on the whistle in Dwarvish runes. If a character succeeds on a DC 20 Intelligence (Arcana or History) check, the character recalls lore that says the duergar made several such whistles for various groups in an age past.

If you blow the whistle in darkness or under the night sky, it allows you to cast the Animate Dead spell. The target can be affected through up to 10 feet of soft earth or similar material, and if it is, it takes 1 minute to claw its way to the surface to serve you. Once the whistle has animated an undead creature, it can't do so again until 7 days have passed.

Once every 24 hours, you can blow the whistle to reassert control over one or two creatures you animated with it