Demonic Boon of Marilith

D&D 2014 cult or boon: Demonic Boon of Marilith. Source: Mordenkainen's Tome of Foes. category Demonic, kind Boon Demons of sufficient cunning and power can bestow boons, using the boons' recipients as pawns. A marilith can grant the following special trait: Serpentine Reaction This creature can take an extra reaction each round, but this reaction can be used only to make an opportunity attack. Boon

Source: Mordenkainen's Tome of Foes, p. 32 - Licenses and attribution