Kronos

Kronos was the former ruler of Hercules' world before he was overthrown by his son Zeus. He was first mentioned in the Hercules animated series episode Hercules and the Drama Festival.

Hercules (animated series)
Throughout the series only minor mentions of him are made. It wasn't until the episode Hercules and the Drama Festival that he would act as the force behind the plot.

Past
After he found out one of his newborn children would overthrow him, he began swallowing them one by one except Zeus who managed to escape thanks to his mother who instead tricked Kronos into eating a rock thinking it was Zeus. Later Zeus returned and freed his brothers from his father's stomach, along with the rock Kronos swallowed. As an act of revenge, Kronos put a curse on the rock so that any God that comes near it will be placed in eternal sleep. He was then imprisoned in an unknown region of Tartarus.

Hercules and the Drama Festival
In the episode, Pain and Panic reveal the Kronos Stone to Hades while singing the story of Kronos to him. They then tell him that he can use it to overthrow Zeus.

Kronos Stone Song
Despite being a minor Disney villain, Kronos is one of the few that has a song sung about them. The song in question is appropriately titled Kronos Stone and is sung by Pain and Panic.

Back when kronos was king of the universe,

he hocked up a rock and gave it a curse,

and now any god that gives it a peep,

falls into a permanent beauty sleep.

Yeah they drift away to the great unknown,

when they take a little peeky at the Kronos Stone.

No need to count no sheep you're in a sleepy zone,

when ya take a little peeky at the Kronos Stone.

Do ya feel a little freaky 'bout the Kronos Stone?

Are your knees a little weaky 'round the Kronos Stone?