Theano was one of the fifty daughters born to Danaid and Polyxo. She along with her sisters (and half-sisters) were married to sons of Aigyptos (by his various wives). For Theano her spouse would be Phantes. That night, Theano as well as forty-eight of her forty-nine siblings killed their husbands.
When Theano and the other Danaids who had murdered their husbands they were sent to Tartarus and were forced to carry water from the river Styx in a bowls (or jars according to some) with holes over to a tub which leaks at multiple parts. Theano and the others were told only when they filled the tub could they wash away their sins. The Danaids continue this essentially impossible task to this very day.
Danaus & Polyxo
"The twelve sons of a Naias Nymphe named Caliadne [and Aigyptos] drew lots for the daughters of Polyxo, also a Naias [and Danaus]. The sons were named Eurylokhos, Phantes, Peristhenes, Hermos, Dryas, Potamon, Kisseus, Lixos, Imbros, Bromios, Polyctor, and Chthonios; the girls were Autonoe, Theano, Electra, Cleopatra, Eurydice, Glaukippe, Antheleia, Cleodore, Euippe, Erato, Stygne, and Bryke."
- Apollodorus, Library 2.1.5