Disclaimer: I do not own Supernatural. I own only those original characters that are in this story.
Dark Meditation
One moment, Sefira had given the ultimatum of ultimata to her uncle. This did the Lesser Darkness do in the midst of her mother, the Darkness: demand that God himself fall down before her, even without venerating her worshipfully. She who arose in might coveted a dark anointment of her head by him, while she herself sat on a throne.
The next moment, the self-proclaimed Active Divine Feminine found herself banished, like a mere angel, from the Divine Presence that was her mother and her uncle, despite having become over three times more powerful than her late predecessor, Death, at the peak of his power. Despite having become over eleven, over twelve, even over thirteen times more powerful than either her eldest primordial cousin, Michael, or her darkest archangelic creation, Samael, and despite having become over nine times more powerful than the latter's dark offspring, she could not withstand divine banishment.
For a short while, the most powerful created being in existence struggled as if she were a banished angel. Eventually, all her temporarily negated power found its way back to her being, and she pondered her next moves.
For this meditative purpose, the Dark Herald created a pocket universe in an instant. She then travelled to that dimension to engage in very dark meditation, for time there passed by much more rapidly and could not be slowed down by anyone but herself, her mother, or her uncle.
There, the created dark essence gazed upon her prize, the enormous Hammer of Death, very distinct from her Instruments of the Darkness which she had either upgraded or created. Then, a new possibility came into her consideration, for she had not undertaken the feat of bringing into existence a spiritual realm like the Commonwealth of Heaven, the independent Republic of Heaven, Purgatory, Hell, the Bottomless Pit, or the Empty. Nonetheless, she desired something separate from the realm of her Reapers, and so she wondered about stretching the Hammer's creative abilities regarding primordial beings and substances, towards the creation of an entire spiritual domain. Darker thoughts about the details of an appropriate domain then came across her mind.
Satisfied with the results of her dark meditation, Sefira went forth from her pocket universe and folded it out of existence. The Lesser Darkness was bound to create a far more durable and permanent domain.
Author's Note: Recapped below is the scope of Sefira's mid-tier nigh-omnipotence, over and above the abilities associated with lower-tier nigh-omnipotence:
Innately - destruction per cataclysm of an entire polyverse, awareness of mind-reading attempts by the Darkness or by God, complete immunity from trapping except by the Darkness or by God, use in normal time of primordial beings to "create" primordial beings or substances from other primordial substances, unrestricted interdimensional transportation, full possession of a supermassive Leviathan gestalt, advanced healing in normal time of any supermassive Leviathan gestalt or any high-tier arch-Naphil or any high-tier archangel, instantaneous obliteration of any supermassive Leviathan gestalt or any high-tier arch-Naphil or any high-tier archangel, instantaneous exorcism of any supermassive Leviathan gestalt or any high-tier archangel, instantaneous obliteration of any spiritual realm, mid-tier umbrakinesis, immediate annihilation of any human soul or any other seemingly indestructible soul, creation in normal time and ex nihilo of non-primordial celestial beings, complete immunity from archangel banishment, complete immunity from Holy Fire, and inherent power negation of the short- to medium-range use of any Hand of God or of any Tablet of the Word of God;
Mainly augmented but requiring recharge - destruction per cataclysm of an entire multiverse, manipulation of time across the omniverse (all Creation) to almost a standstill (not yet learned), true creation in normal time of one primordial being or substance (not yet learned), and creation in normal time of any spiritual realm (not yet learned); and
Fully augmented but requiring recharge - destruction per cataclysm of an entire ultraverse or collection of multiverses, alteration of all rules of existence except time itself and the mandatory existence of the Darkness and God (in development), and existence entirely outside time itself (not yet learned).