hades
Yes... beasts, monsters, and the undead
The kindly ones are a nickname for these monsters. These monsters are actualy called the furries and they are monsters that belong to hades. The furies in the Ligthning thief are sent by hades to apparently get the lightning bolt that Percy "stole." But is that what he is being accused of stealing?
Hades had his own realm of which he was King, that of the Underworld, and Hades did not often leave it.
Hades himself owns a beast; guard dog named Cerberus. And I am sure The Underworld contains many countless unusual beings.
shadow
hades power is to rule the underworld and he has it as his own rule
A dorky kid who turns out to be the powerful son of Hades
The Titans? Like, Kronos (Zeus, Posiedon, and Hades dad)?
Hellhounds, Medusa, and Hades (technically, he is not a monster, he is a god)
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.
Hades is married to His own neice, and His brother Zeus is also His Father-in-law.