A Timeless Prison in-game; both the device and the dome are visible

Timeless Prisons 
are angelic devices found in Act V of Diablo III.

These constructs, when activated, project a blue dome of energy, trapping anything that was in the area at the moment it was activated, freezing its victims in time. In this state, they are impervious to physical harm and completely inert. Interestingly, objects that come into the dome after it has been activated, are not affected and not put into stasis. Deactivating or destroying the Prison causes all of the trapped victims to awaken shortly, without any idea that they even have been trapped in the first place. It is possible that Timeless Prisons were intended to keep their victims imprisoned forever, in order to prevent them from being reborn in Burning Hells after death.

Erelus's example shows that entities other than demons can be frozen in time inside of them, angels included.

A number of Timeless Prisons were deployed in Battlefields of Eternity and Pandemonium Fortress, serving as eternal prisons for the powerful demon champions. In particular, they were used to trap Siege Runekeepers Volux the Forgotten, Emikdeva, Korchoroth and Rekkar in order to prevent them from activating the Ram. Along with their soldiers, they were kept in Timeless Prisons until the Nephalem deactivated them, killing the demons and claiming their Siege Runes. One can also find several Prisons in the Fortress, containing powerful demons such as Amok the Breaker and Inquisitor Sebacious.

The Cursed Crystals are a completely different method of imprisoning demons, although these serve the same goal and with roughly the same result.

Interestingly, some Prisons must be destroyed to be shut down, and some must be manually deactivated by touching them.

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