The Grimoire of Natasha

The Grimoire of Natasha is a magical text book that contains the necessary runes to summon a demon. The demon is not required to obey the summoner, but it may. The summoner always has advantage on rolls against the demon. The demon can choose to leave at any time. The runes are effective 3 times; after that, they lose potency and the demon is no longer necessarily summoned.