"Eternal Sleep" is an ability wielded by multiple villains and monsters in Sailor Moon. It often functions as an tool for punishment used by those in higher positions of power.
The first is the state Queen Beryl put Jedite into when he was defeated by the Sailor Scouts. Jedite was put into an eternal frozen state, then vanished into infinite darkness. Beryl later threatened Nephlite and Zoycite with the same fate.