In FR the dragons were created by the primordials. Does this mean many/most/all dragon gods are also primordials, as aspects/fragments of Io?
The Dragon Gods are presented as actual gods rather than primordials (DMG p. 10, PHB p. 296, & SCAG p. 113).
Can a primordial also be (or somehow become) a god?
I'm unaware of this occurring, but I wouldn't rule it out:
- Asgorath/Io is both, but (as creator of the universe) doesn't count.
- Tharizdun is close, but is not a Primordial.
- Kossuth is also close, but he's "not a true god but actually an
elemental primordial".
Do primordials need, desire or benefit from followers?
Not normally, but Kossuth is an example of a primordial that benefits from extensive worship.
Do surviving primordials have significant religions associated with them, either in Abeir or in Toril?
On Toril the five Elemental Lords have followings, while The Seven Lost Gods were once worshiped. I believe Abeir would have more significant Primodial worship, but that setting was never detailed.
Do primordials grant spells in the same way as gods?
Not normally, but again: Kossuth is an example of a primordial that both grants spells and has extensive followings.
Can they be warlock patrons?
I don't see anything preventing this. A few homebrewed examples of this can be found here and here.
Is there any obvious difference in the portfolios of primordials versus gods? I would have assumed that since they predate mortals they tend to represent more fundamental forces (like the elements) rather than human concepts like law, love, luck, agriculture.
That might be one way to view it, but 4e's creation myth (Worlds and Monsters p.56, cited below) has the Gods being created at the same time as the Primordials. Another way to view it might be that the Primordials are composed of elemental "physical-matter" while the Gods (and Astral Sea) are more composed of thought (mental-matter).
In short, what is the difference between a god and a primordial? Are they fundamentally the same (just different lineages) or are there fundamental differences?
They are consistently presented as having fundamental differences. From 4e's Worlds and Monsters (p.56): "The gods, beings of divine power, appeared in the Astral Sea, while in the Elemental Chaos arose the primordials, incarnations of tremendous elemental might"... "composed partially of creation-stuff".
I also found this fantastically detailed post on Candlekeep...
That still seems like as good a guide as any.
The implications are minor, but not totally zero. To address your specific questions:
The type of spirit does not cause any visible differences, unless the player or the DM want it to. That would be a flavor/reskinning call, not a rules or mechanics call. Note that the spirit type and the shape taken are not linked or restricted in any way. A stark example is if you are a Pact of the Chain Warlock, you familiar can take the shape of an Imp, Quasit, Sprite, or Pseudodragon, and this is given no limitation as to the type of spirit. So you could have a Celestial Quasit.
The type may have an effect on the spirit's alignment, but again, that's up to the player and the DM. Again, that would be a flavor/reskinning call, not a rules or mechanics call. It obeys your wishes and commands regardless. It would be up to him to decide if his Celestial Quasit is mannerly, polite, and never uses profanity.
Yes, the default is that the character gets the same spirit back, just in a new shape. The caster does have the ability to dismiss a familiar forever, so presumably if he does so, casting it again will get him a different spirit, and could choose a different type. Again, flavor/reskinning issues, up to the player and the DM. Certainly, if the DM allows him to change the spirit, getting one of a different type would mean a new spirit, and thus a new NPC, with no memories of what the last one experienced.
The main actual effects, as far as the rules go, are that the type of the creature is celestial, fey, or fiend, (and not beast), and this may affect what spells it is affected by (e.g. immune to Animal Friendship, but may be subject to Magic Circle. Also, depending on its type, a Paladin's Divine Sense may detect it, and that could tell them something about the character (or at least who he associates with).
The point of it being a spirit, from a mechanics standpoint, is mostly to explain where it comes from, why it is obedient and maybe more intelligent than normal, and to provide a mechanism for summoning, re-summoning, and dismissal.
How much control the character has over the form of the familiar is, again, up to the player and DM. If I, Gimble the Great, decide I want an owl, is that all I can specify, and the spirit decides if it will be a 4-ounce burrowing owl, or a 5-pound snowy owl, and all the details of its appearance? Or could I specify the species of owl, but it gets to decide the coloration? Or do I get to pick all the details myself?
Best Answer
This type of creature is not described in the rules
After sifting through all of the celestials and all of the fiends, it appears there is no mention of a pair that could procreate in such a way to create a part-celestial/part-fiend individual. You could, however, create such a creature using the Dungeon Master's Toolbox in the Dungeon Master's Guide where it details how to create custom monsters and custom races (both of which could be useful in your friend's endeavor).
What we can find for celestials
In 5e so far, there are very few celestials mentioned in the rules. Even still, there are some creatures that seem promising. The only celestials that mention a mechanism for reproduction are the Couatl and the Pegasus (both found in the Monster Manual):
Couatl procreation is described as
I could easily see a fiend that reproduces through infecting the eggs of non-fiends. Such a creature might infect a couatl egg and the resulting creature may share some of the features from the original parents and the infecting fiend.
And the Pegasus entry simply states
This is pertinent as a nightmare (a fiendish mount) is created by
It is unclear if this nightmare would still be able to reproduce with their former mate, and what would result in such a union (if it were somehow facilitated).
What we can learn from fiends
There are a number of fiends that are born from the union of fiends and humanoids. Any of these creatures may resulting in semi-celestial origins if the humanoid shared celestial blood. The features of this type of creature (such as whether they remain celestial in any way) are also unclear. You can find a list of humanoid-spawned fiends below:
MM = Monster Manual ; VGtM = Volo's Guide to Monsters
Using any of these creatures, you may be able to get some guidance into how a part-celestial, part-fiend would come about.