There are no 'planes' or 'planar creatures' as traditionally understood. There are no gods and no strong evidence of gods, although there are powerful magic relics (indicating very powerful magic in the distant past) and a few powerful beings that may portray themselves as gods to mortals that otherwise may not understand what or who they are, but are not 'gods' with the power of gods.
The Fey is a magical and wild distant place where time seems to flow differently. Characters who know of it do not think in terms of "planes" but more as a distant land (perhaps a different continent) that they teleport to, it is not quite 'the Feywild.' Traveling through a Feygrove during a full moon seems to transport someone to the Fey, and traveling through that area in the Fey during a new moon seems to transport someone back. The only place known in the Fey is Mithrendain, the ancient and powerful city of eladrin that has a strong defensive enchantment surrounding it. Mithrendain consists of 5,000 eladrin, and is larger than any single settlement known in the Nentir Vale. In Mithrendain, the sky is permanently twilight.
The "Great Deeper Dark" is a shadowy place where Kendred hypothesizes that souls of the deceased go after they die. It is not 'the Shadowfell' and does not serve as home to many independent-minded conscious beings. It is closer to being a shared condition during the transition of a soul from the mortal body to permanent death. Some 'demon-like' or dark creatures have communicated through this Great Deeper Dark. At the Library of Highforest 200 years ago, the necromancer Greldin contacted such beings, as did the more recent necromancer-scholar Ulferth. The creature taught Ulferth how to trap souls in orbs for permanent storage, and the creature used Ulferth to overrun and destroy the Library with undead creatures. On a body's death, the soul departs and travels briefly in the Great Deeper Dark until it is lost from all knowledge or communication, which is why there is only a limited time to ressurrect someone after death. On a body's death, the animus (life-energy which provides a body's movement) also departs the body and travels into the Great Deeper Dark, but much more slowly and sort of leaks into that place. This is why necromancers figured out how to bring back long-dead corpses with movement-energy (some remaining animus that hadn't leaked out yet) easier than bringing back long-dead souls. Also, this means there are no undead with souls or minds..
The necromancer cult's goal seemed to be figuring out how to permanently trap a soul in a body powered by animus so they could live in a natural body forever. Bulwark's body is powered by magic, not animus, and the soul seems to suffer a wiped memory when placed into that sort of body, but otherwise his permanent existence is their goal. The necromancer Ulferth seemed to have very recently achieved the soul trapping magic from the dark creature, and the necromancers already had the magic to bring both a soul and animus from the Great Deeper Dark and into the corpse (this is how the party learned to do this/res). The only thing the necromancers could not do was ressurrect long-dead souls as they had departed the Great Deeper Dark already and been lost. Ressing, or pulling souls back from the Dark and putting them back into a body, only seems to work for a short while when the soul is still in the Dark. After that, souls seem to disappear from the Dark and ressing won't work. Animus, however, can still be accessed for a long time.
The Far Realm is also not a plane (no 'planes'). The term 'Far Realm' is used only to signify the maddening infinite space and the dark, powerful, and hostile creatures that lurk there outside our own world. Their power is often beyond magic or technology and even reality. The more one understands them, the more one loses logic, mental stability, and coherence. Often for some reason these distant space-creatures cannot simply use their powers to teleport the very great spatial distances to our world, and much animus-energy is needed to facilitate their transport to and from our world. This is why creatures from the "Far Realm" are often in contact with necromancers and others who work through the Great Deeper Dark. This is why those creatures harvest animus-energy for their spellcasting and powers. So these creatures seem to work through the Dark to try to convince necromancers to harness animus/life energy so they can come to our world, for unknown but likely hostile reasons.