Soul Crucibles (a.k.a. Death Orbs) were arcane devices employed by the Reapers. Only the Death Maidens had power to operate these constructs, and even they could not create more in the field. Visually, Crucibles appeared as black orbs sucking in energy, with twisted, agonized faces of tormented souls periodically emerging from their surface, only to be drawn back inside.
Crucibles varied in size and power, with the smallest ones connected to the so-called Master Crucibles. If a Master Crucible was destroyed, the Death Maidens would not be able to create more (according to Myriam Jahzia, at least). Crucibles were, in fact, soul traps, absorbing and storing the soul energy from nearby corpses and surroundings, leaving nothing but soulless husks. This energy could later be used by the Reapers for various activities. Numerous Crucibles were deployed in Westmarch and destroyed by the Nephalem.
Larger soul energy vaults could be found in the Pandemonium Fortress. Jamella mentioned that Soul Crucibles had limited capacity, and were to be emptied into the vaults (Soul Prisons) when full. One such Prison vault must be broken for the player to acquire the power of the dead.
The largest crucible was at the Heart of the Fortress. The reformed Black Soulstone worked in a similar manner, but had unlimited capacity, absorbed demonic souls only, and could leech souls even from living beings.