Vestige Domain

Periodically, deities die. Usually, this is at the hands of other deities or powerful primordial forces, but occasionally the death is caused by mere mortals. Regardless of the cause, the ramifications of a deity's death are far-reaching. If another deity is able to quickly pickup the lost portfolio, the disruption to the realms can be contained, but when an entity in opposition to the deceased deity is left in control catastrophic disorder is in store. 

Each deity maintains a realm in the afterlife, a place reflecting the outlook of the god and its worshipers, where they can exist in eternity. Almost. When the deity dies, the realm becomes unstable. A friendly deity coexisting on the plane can ease the cosmic tremors, but in the absence of such divine intervention, the realm will collapse in upon itself and be transported to the Astral plane, adrift as the corporeal remains of the dead god. The god-corpse exists in direct correlation to the power of the deity, the number of followers it had, and the length of time it ruled its domain. A young demigod corpse might be a few dozen meters long - insignificant on the cosmic scale. A greater deity of considerable longevity might yield a continent or planet-sized corpse adrift in the sea of stars. These corpses become havens for Githyanki and other plane-travelling outsiders, moreover to the still-living worshipers of a dead god, finding the corpse of their deity is often a key component in restoring its divine spark, and reigniting godly powers.

Inevitably, shortly after the deity is killed, another deity will step in to fill the void and resume granting divine power to the faithful. Sometimes this is an allied power, sometimes the holder of the same or similar domain, and sometimes it is the usurping upstart which caused the death. The replacement deity has little authority over what the absorbed worshipers do with the power granted to them - they are not beholden to the new god, and may actively work against its wishes in trying to restore the older faith. 

Tyche. Amaunator. Jergal. Mystryl. Moander. Torm. Bane. Bhaal. Myrkul. Helm. Mystra. Tyr. Azuth. Many gods have fallen and risen again. The key is retention of faithful worshipers and divine artifacts, through which the divine spirit is reignited and the astral body preserved so that generations of previous worshipers are not wiped from existence. 

Vestige Domain Bonus Spells:

1st    -    False Life, Shield

3rd    -   Gentle Repose, Lesser Restoration

5th    -    Magic Circle, Speak with Dead

7th    -    Banishment, Private Sanctum

9th    -    Planar Binding, Raise Dead

Untethered

Your god is dead or thoroughly dormant, you are not bound by old codes or ethos - as whatever entity grants you powers has no say on what your deity would or would not allow - nor new ones, since you are not pledged to the new deity's service. But your divine spells are weakened, reducing spells dealing necrotic damage, radiant damage and healing to their minimum die roll (typically 1). 

Channel Divinity - Desperation

Your lot is a scramble for second chances, and when you call upon the power of your vestige, the act of desperation is reflected in the boon placed upon you. For 1 minute, as an action or bonus action, you can reroll a single die on your turn. This feature may be used to bypass the Untethered penalty of automatically rolling the minimum possible on necrotic damage, radiant damage and healing spells. Additionally, as a reaction each turn you can force a reroll of one die when the outcome directly affects you.

Nothing to Lose

No afterlife awaits you when you die - your deity has no realm in which you will spend eternity. To this end, at 6th level, you imbue your holy symbol with three contingent spells as a last resort: Gentle Repose, Revivify and Lesser Restoration. Triggered upon your death, your body and spirit are magically preserved for ten days, and you are then restored to life with 1 hit point. You can be raised from the dead by other means prior to this, but your holy symbol must be present for such a spell to work. This contingency does not work if your holy symbol is not in your immediate proximity at the time of your death or at the end of the ten day period, or if your corpse is entirely destroyed (devoured and eaten, immolation, disintegration, etc). There are no guarantees that there will be no hazardous conditions upon your rising - if you are afflicted with multiple diseases or buried underground or underwater it is unlikely you will survive long the second time around - and you must recast all three spells to imbue your symbol with this protection again. You could provide your symbol as a boon to someone else in an extreme circumstance, but you may only have one such contingency at a time and take a terrible risk by giving yours away. 

Echoes of the Dead God

When your weapon strikes an enemy there is a hollow echo which builds in volume to damage your foe. At 8th level your weapon attacks deal an additional 1d8 thunder damage on a hit. This damage increases to 2d8 at 14th level. 

Astral Shelter

You have discovered the location of your deity's corpse in the Astral plane and can use it as a refuge. At 17th level you may cast Plane Shift or Gate to reach this location and begin the process of restoring your deity to life anew. You may have been transported here by a higher level cleric and entrusted with the location, or you may transport lesser clerics and faithful here, but cannot yet entrust them with its location. 

The demiplane is gigantic, but is initially barren and nearly featureless, like dry, desiccated corpse-skin. Repeated castings of the Hallow spell and similar magic can eventually restore life to the deity, but the time it takes to do so will exact a heavy toll on any creatures who remain on the plane long enough to accomplish such a feat. There is a time-distortion between this plane and the Prime Material Plane (and most others operating on roughly the same time scale) of 48 : 1. This means 1 turn is equivalent to nearly 5 minutes on the astral demiplane, 2 turns is 10 minutes, 10 minutes on the Prime Material Plane equates to 8 hours on the astral demiplane, 1 hour is 2 days, 1 month is 4 years, and so on. Entire generations of priests could live and die trying to resurrect the corpse of their deity, while only a year or two passes for those they left behind on their home plane. For this reason, efforts are typically made to recover powerful religious artifacts which could speed up the process. 

 Utmost care must be taken to keep the astral body safe and secret from powerful creatures (including fallen Celestials, Demon Lords, Archdevils, Dragons, Gith and similar beings) which can use the divine corpse as a morphic demi-plane to springboard their apotheosis and ascend as a divine power. Any single creature with a CR of 20 or greater which spends an hour attuning to the morphic qualities of the plane can alter nearly any environmental feature (including the time distortion) within a radius equal to its CR in feet, expanding hourly until the entire demi-plane is changed to fit its desires. Once this change is complete, the deity's corpse has ceased to exist and cannot be restored by any means, and the demiplane is likely sucked from the Astral plane to a location elsewhere in the cosmos more befitting its new master.