Agave is Pentheus' mom and Dionysus' aunt. Unfortunately, when Dionysus shows up in Thebes, it is not a happy reunion. Agave and her sisters Ino and Autonoe have spent the years since Dionysus' birth spreading dirty rumors about his mom Semele, who was also their sister. Rather than admitting that Semele was impregnated by Zeus, Agave went around telling everybody that it was some random mortal guy. Why did Agave and her sisters do it? We don't know. Maybe she was jealous that Zeus didn't pick her. (Not that that would've been all that cool, since Semele was obliterated for the affair through a trick of Hera.)
To punish Agave for her gossipy ways, Dionysus makes her go crazy, turn into a Maenad, and lead the rest of the ladies in town in the wine-soaked rituals up on Mt. Cithaeron. The final punishment comes when Dionysus makes Agave think that Pentheus is a lion. Fueled by wine and Bacchic fury, Agave rips the head from her own son's body. Dionysus only allows his aunt to come back to reality when she's back in Thebes with Pentheus' head on the end of stick. Needless to say, this is a pretty awful moment for Agave. We have a feeling she won't be spreading nasty rumors again.