The principal subject of the The Bacchae, Dionysus, possesses a multitude of powers and can take a variety of forms. In Euripides's conception of the god, however, his numerous forms conform to the logic of duality, that is, they are both one thing and its opposite simultaneously. Thus, Dionysus is presented as being both inside and outside the play's action. Physically, he is both beautiful and fearful. By birth, he is both divine and human, the son of Zeus and a mortal woman. By origin, he is both Greek and Asiatic, his cult associated with Asia Minor. His cult names provide insight into the nature of his relationship with humankind: the bacchants call him Bromios, "the roaring one", and also Lysios, "the god of letting go". Dionysus's gifts allow humans to let go of their troubles through wine, to let go of their identities through theater, and to let go of their individuality through cult worship. For humans, his ability to allow them to let go, when practiced in moderation, opens them to the festive, communal side of life. As the Stranger, or Dionysus's human form, says, "Dionysus, as a god in perfect essence: a terrible one, but to men most gentle."
But this letting go, like everything associated with Dionysus, also has its potential dark aspect. There is no inherent limit to the powers of bellowing Dionysus. Festivity can turn to destructive excess, and instead of providing a necessary temporary release, can overpower life itself. Without self-control, Dionysus's powers can drive humans to let go of their sanity, to let go of their judgment and, in the end, to let go of their very humanity. The supreme importance of self-control is embodied by none other than the disguised Dionysus. While Euripides vividly illustrates the full extent of Dionysus's ecstatic powers upon his followers, the Stranger himself is calm, self-possessed and patient. He alone displays self-control and wisdom, and these traits distinguish him from the mortals who surround him. While he is able to sting men with madness, he is the picture of sanity, suggesting that he is not the agent of the tragedy, but perhaps that mortals themselves are responsible for their bloody discord.
Euripides builds the principal dynamic of The Bacchae around the conflict between Pentheus and Dionysus, and sets up several interesting parallels between them. Pentheus, the King of Thebes, and Dionysus are both grandsons of old Cadmus, but while Pentheus is his chosen heir, Dionysus is not even recognized by the king, nor allowed in the city. They are both young and powerful and want to establish their authority over Thebes, but the kinds of authority they want to erect conflict with one another. Pentheus wants to establish an earthly, rational authority as the single legal sovereign, so much so that he adamantly refuses to allow even the worship of Dionysus. Dionysus wants to establish his joyful divine authority over the city and contest Pentheus's project of a purely rational civic order. As guardian of social order, Pentheus is repulsed by the cult and disturbed by the idea of women roaming freely in the wild, for the order that Pentheus represents is not just the legal order, but the proper order of all of life, including the supposedly proper control of women. Dionysus is seen as threatening all this, and so Pentheus states that the "effeminate stranger is introducing a new disease for our women and dishonoring their beds." Pentheus continuous refusal to accept Dionysus leads to his downfall. But even though he is obsessed with law and order, Pentheus is also shown to be vain, obstinate, suspicious, and arrogant. The old seer Tiresias describes Pentheus's principal fault well when he says, "do not be too confident that / sovereignty is what rules men nor if you hold an opinion, take that opinion for good sense." In the end, Pentheus too falls victim to Dionysus's madness, and lets his illicit desire to see the maenads carry him to his death.
It was a common convention of Greek theater that distant, violent, or complex actions were not dramatized, but rather took place offstage and then were described onstage by a messenger. In the case of The Bacchae, this convention is used to bestow even more power on the already fantastical events and also to grant a certain respect to the Dionysian rites by not showing them directly. Indeed, the actors and audience find out about the practices of the bacchants principally through messengers, who often arrive out of breath and frightened. The use of the convention thus accords with historical practice, since, as one of the mystery cults that flourished in Greece alongside state religion, Dionysian cults required that their rites be kept secret from outsiders. When Pentheus asks the disguised Dionysus to tell him what the cult's worship consists of, Dionysus responds, "they may not be uttered to those of men who are not bacchants." Messengers provide the means both for preserving the mystery of the bacchic worship and for making it vividly real for the audience.