Source: Quests from the Infinite Staircase

Wondrous Item, Rare (Requires Attunement)

You can use an action to blow this horn to cast one of the following spells from it: Arms of Hadar , Fog Cloud , Gust of Wind , or Web . If the spell requires a saving throw, the spell save DC is 13.

Once the horn has been used to cast a spell, it can’t be used to cast that spell again until the next dawn.