Requilac... additional comments. Apologies...sorting out the quoting mess will not be fun.

Spoiler: Further Deep Spawn Comments
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You know, I was actually taking some of that into consideration. To me I thought it had a really solid and concrete, but I was worried that other people wouldn't be able to pick it up. I have a lot more to say about the comments above, but I will mention those later.
Thanks for kindly not mentioned that I referred to your class by an incorrect name throughout (Deep Dweller versus Deep Spawn)…/ahem I think this “issue” may stem more from my own limitations…In regards to the theme confusion, Deep Spawn conjures (to me) something coming from the deep oceans or waters and so I locked myself into this motif.

Yeah I noticed that pretty early on. Keep in mind that for the most part the archetypes are largely there for flavor with little mechanical impact though. I know that some people may not like that idea, but I personally found it the best approach to go with for this class.
Understood. That makes sense now. Given that this is the case, there still should be some balancing between them as there are mechanics involved.

As for the martial weapon proficiency, I do have two reasons behind that, both of which I discovered when I was actually making this class' predecessor, the Aberration martial archetype for fighters. First off, some people don't like the imagery of using natural weapons. They think it looks odd, especially the maw, and struggle to visualize it. Some just prefer the imagery of using normal weapons, and giving them proficiency in martial weapons doesn't make the class overpowered, so I added it in. My second rationale behind giving them martial weapon proficiency is in case they come across any magical weapons. It’s going to suck if you find a Sword of Life Stealing or Trident of Fish command but can't use it because you lack martial weapon proficiency after all.
Good points. I was coming from the perspective that since the natural weapons provided by Unnatural Combatant were always best case (in regards to martial weapons), that the martial proficiency was largely redundant but people like choices (especially if they find a choice magical goodie).

That is all true, its just that I have this weird quirk of disliking the idea of my more alien sub classes to be easily harmed by mental effects. It just feels wrong to me that this embodiment from another plane of existence would be scared half to death of a dragon or gawking over a dryad. But in truth, constitution makes much more sense. I am going to get over my qualms and switch proficiency in wis saves for con.
You could always add in advantage to saves versus fear as part of the core class…

I can't possibility account for multi-classing into this equation. I am trying to make this class as best as it can be in itself, if i take into account multiclassing makes things become much harder than they need to be.
Ha! I grumble about the same thing when someone brings it up on my own homebrews (and trying to account for feat interactions as well)… I guess I’ve heard it enough that it’s now a habit to critique for it.

Of course the Deep Spawn would be proficient in them, they are called simple weapons which the class has proficiency in. I will make it cost a bonus action if you feel it should come up. It doesn't change much either way, and I personally don't think it matters all that much.
It can matter quite a bit in how the class interacts with other classes (the multi-classing dirty word). Also, thematically, I like the vision of these appendages “slowly” (in the time of a bonus action) growing/forming rather than just instantly being there.

See my above reasoning for why I kept martial weapon proficiency. The only reason that I considered the unnatural weapons simple is because I think that makes more sense thematically, even if mechanically it looks a little strange. It should be easy for the Deep Spawn to wield them after all, they become part of their body. I would find it even weirder if they were considered as martial weapons.

And wait a second, they don't deal magic damage? They don't do they? That was completely my mistake, they were supposed to, but I forgot to add that in. Thanks for catching that mistake.
Np. The benefits of multiple eyes on your work.

The second sentence explains all of your questions; "This vulture cannot interact with the world (or be interacted with) anyway other than what is described in this feature." The invisibility is just there for fluff. I don't want to have it cost an interaction, because the deep spawn already has too much competition for its bonus action already.
This somewhat confuses me. As far as I can tell, the Deep Spawn has no feature other than the familiar that use its bonus action (other than the proposal for Unnatural Combatant)? I always tend to favor having to make choices (e.g. via what can be done with action economy) than allowing freebies. Just a design philosophy.

And also I am going to be completely honest with you; this feature is basically just a more thematic version of the Sahaugin's Blood Frenzy. In fact originally they did just have Blood Frenzy, but I wanted something with a little more oomph.
Not familiar. I’ll check it out.

I don't believe I fully understand your case with saying that the vulture isn't thematic though.
It’s not that it’s isn’t thematic, it’s that I don’t understand how it ties to the Deep Spawn. A (in my mind) deep sea dwelling abomination having a vulture… just doesn’t resonate.

This class has a recurring theme of the twisting of nature and animal imagery. Deep Dweller makes you amphibious like a fish. Unnatural combatant gives the armor and weapons of an animal. Tongue of the Deep allows you to communicate with animals. The Spirit Familiar can turn into an animal. Elemental Body is reminiscent of an electric eel.
Worded like that, it makes more sense. Perhaps I was being too narrow in my own interpretation.

And also, keep in mind what a vulture symbolically represents. They are thematically tied to corpses, mortal injures and the dying, necromantic magic, the afterlife and souls of the dead, etc. Altogether some vile imagery, which gives the Deep Spawn a more sinister mood. Really emphasizes the anti-cleric, ant-druid and anti-paladin mood a lot.
I agree… it’s just hard for me to tie the Deep Spawn (ocean, water) theme to a traditionally arid/savannah based carrion bird. Again, probably my own issue for mentally locking in to my own conception of what the name “Deep Spawn” evokes.

It is fluff more than anything. I just noticed that when designing the spell list, a lot of fire spells came up (burning hands, hellish rebuke, fireball, scorching ray) and wanted to make the theme more consistent.
Yeah, it really works well with the class allowing this change. If you wanted (though I don’t know why you would) to go a step further in locking down the theme, you could just state that any spell that inflicts fire damage instead inflicts cold damage.

A creature is frightened until the start of their next turn, as described earlier with the following clause in the first paragraph; "Any creaure that has a direct line of sight to you which moves to a location within 10 ft. of you or starts its turn within 10 ft. of you is subjected to one of the following effects until the start of their next turn."
Reading comprehension fail on my part, eh?

How does this remind you of rage exactly? Barbarian Rage allows you to use a bonus action to gain advantage on strength checks and saving throws, deal extra damage, and gain resistance to B/P/S damage which last for 1 minute. It has multiple charges which are restored on a long rest. The Deep Spawn's Hideous appearance allows you to use a bonus action to generate an aura effect which debuffs enemies and last for 1 minute. It only has one charge but it is restored on a long rest. Literally the only similarity I am seeing is their durations and the fact that they regain uses after a long rest, but that's hardly an original mechanic.
I didn’t mean as an exact mechanical copy, more as an initiated 1-minute long combat state buff. Maybe a stretch, but as I tend to design based off other abilities, I’m always looking for the baseline/inspiration.

It does not advance. It's just a familiar, its not really supposed to be used in combat except in the most niche of scenarios. Its a CR 1/4 monster, so according to Xanathar's its like having a 1st level PC along for the ride. And the reason that the familiar can turn into an invisible and usually tiny animal is what keeps it from being a liability.
Whereas it’s certainly structurally based on find familiar, the options seem to have a little more “oomph” than the standard familiar choices. I just made an assumption that this was intended as a – albeit minor – power boost to the melee capabilities. It might be a good tweak to the feature/class to allow some minor advancement of this creature (e.g. add your proficiency bonus to its AC, to hit, damage, and increase its HD occasionally)?

Yeah, I think I do need to secure its theme a little bit more. This should just deal cold damage. As for the damage bump, I was basing this feature off of the paladin's improved divine smite, which deals an extra 1d8 radiant damage on a hit, but doesn't have the ability to seize hit point regeneration.
Speaking of smite… could you see allowing the burning of the Deep Spawn’s spell slot to maximize this (rather than a blanket progression)?

I think it is actually really thematic. It really seals the deal that the Deep Spawn is an unholy and dark thing.
Oh, it’s completely thematic and fitting. It will be frustrating to a player to be given a flaw rather than a feature upon levelling though. I don’t think it would less thematic (or really detrimental) to include it in the Deep Dweller feature which would open up some design space to include a feature that is helpful/beneficial to the player at this level.

I wanted the lightning damage there to make it seem reminiscent of an electric eel. That being said, I see your point, and I will just make it cold damage.
Now that I see your vision better, I understand all the elemental choices.

I think it actually fits the theme well here well. As I previously stated, a rec curing theme here is the subversion of nature and the natural order. Deep Dweller gives you inhumane characteristics, Unnatural Combatant gives you features no human should have, your magic itself is not native to the Material Plane, Hideous Appearance makes you so wretched looking that it actually harms others, your spirit familiar is clearly some necromantic/summoning ability (neither of which are wholesome to the world), Necromantic Weapons prevents someone from healing, and Weakness to light makes it clear you are enemies with the preservers of nature.

And with Weather Manipulation, you take one of the strongest embodiments of what should be natural and make it unnatural. Snow in deserts, scorching flood inducing heat in the tundras, drought in the rainforest... things that should not occur, but which do because of the deep spawn's malign influence. What should be natural is their toy to mess with and use for the torment of others. I also like how it reinforces the idea that the deep spawn's very presence inflicts a "plague" upon the world. Long before you see it, you will feel the impacts which the deep spawn makes upon the environment. Very creepy ability.
Point taken (and understood)… just doesn’t feel very impactful (other than from a RP/thematic sense) as a capstone.

That and Gothic and Lovecraftian horror does make a lot of mention of storms and ice, so I thought it would be a nice to allusion to some of those pieces.
This really had a Lovecraftian feel (Dagon, Innsmouth)… it also reminded me of the old movie, Conan the Destroyer (where the “monster” was also Dagon-esque).

Overall, shaping up to be a fun class. It’s certainly a much less literal take on fear that I took.

Now, to find the time to give some comments on the other class in the contest…