Deception, especially disguise and mistaken identity, are at the core of the plots of both Twelfth Night and The Bacchae. In Twelfth Night, Viola deceives Duke Orsino and the countess Olivia by disguising herself as a man, Cesario. Comic mischief erupts in this gender-bending play as the Duke, who Viola/Cesario has fallen in love with, uses Viola/Cesario to woo Olivia, while Olivia falls in love with Viola, thinking that she is a man.
In The Bacchae, Dionysius uses deception and disguise for a more sinister purpose to destroy the king of Thebes, Pentheus, who wants to stamp out the orgiastic bacchanals of Dionysian worship in his city. Dionysius tricks Pentheus into thinking he is a mortal, casts a spell on him so that he mistakes a bull for Dionysius, and convinces Pentheus to cross-dress as a woman to see other women in Dionysian worship. Dionysius betrays Pentheus by putting him a tree, then telling the worshipping women that an enemy is there. In a trance, the women, including Pentheus's mother, tear Pentheus apart, thinking that he is a lion. Thus, Dionysius rids himself of a problem.
While both plays centrally use deception and disguise, in Twelfth Night, Viola uses disguise to protect herself rather than for malevolent purposes. In this zany romantic comedy, genders realign, love triumphs, and the worst thing that happens is that Malvolio is deceived into acting insanely—and is carted off to an asylum. Nobody dies, and everyone receives their just desserts. In The Bacchae, Dionysius's deceptions lead to a tragic end in the death of Pentheus at the hands of his mother and aunts. Agave is cursed and sent into exile despite not knowing what she was doing when killing her son. Dionysius thus gets revenge on a family of his cousins who have not believed him to be a god.