Zeus sends Hermes, the messenger-god, because he's the only one who regularly goes there on business (Hermes guides souls to Tartarus).
Zeus sent the god Hermes to intervene in the myth of Demeter and Persephone. Hermes acted as a messenger between Hades and Demeter, helping to negotiate Persephone's release from the Underworld.
Zeus sends Hermes to make a deal with Demeter to persuade her to let the crops grow again in order to end the famine caused by her grief over the abduction of her daughter Persephone by Hades.
When Hades saw Persephone he thought she was beautiful and he asked Zeus for permission to make her the queen of the underworld. Persephone was Innocently picking flowers when Hades burst up through the ground, grabbed Persephone by the waist and pulled her down into the underworld. When Demeter was told by Helios the sun who sees all what had happened and Demeter morned for her daughter. Demeter sent word to Zeus that as long as her daughter was gone the earth would bare no fruit. Meanwhile Persephone did not eat anything out of spite towards her captor. People were dying due to starvation and eventually Zeus had to send Hermes to get Persephone back. When Persephone received this news she was so thrilled that she ate six (four, seven or eight depending on what version) pomegranate seeds. Having eaten in the underworld she was bound to the underworld (just one of the rules) so she spent six (four seven or eight depending on what version you read) months in the underworld and spent the rest with her mother. During the time Persephone is gone the earth is bare and fruitless (winter/fall) but the earth thrives when Persephone is with her mother (spring/summer).
Zeus did not "send Hades to the Underworld"; just as Zeus rules the heavens, Hades rules the Underworld. According to one of the myths, Zeus selected the Heavens as his realm, and allowed Poseidon to select next because he was eldest. That left Hades (or more accurately Plouton) the Underworld as his realm, which he found suited him just fine.
Demeter, her mother, demanded it: or no living thing would grow. Now half the time Persephone is in the Underworld it is winter, and when she returns it is spring.
Zeus, Hades, and Poseidon, or the Big Three, swore on the River Styx, to no longer have demigod children, so when Zeus had Thalia, he broke his promise. Hades, angered that he broke his promise, sent horrible monsters after her.
Persephone, being Queen of the Underworld and the dead, did not kill - but she could send plagues as she and Hades did upon Thebes when they would not bury the dead warriors after the Seven Against Thebes war.
Hermes leads the dead down into Hades.
no
To the Underworld, and levels therein.
Hades sent the Minotaur to abduct Percy's mother.
Hermes.