I really like the concept. It reminds me of both Voldemort and the Witchblade, in a sense.

Awesome Design.

Great subclass if you want to play as Eddie Brock, not Venom.

@Serigala As this is a homebrew, there is no "official" channel to apply it such as through D&D Beyond. However, feel free to play test it in your own games and tell me how it goes! Just give credit where credit is due! This was a favorite of mine to do and its a unique subclass that me and my friend are really proud of.

@Bjornvetr I can see that actually. The definite inspiration was Venom and other direct parasite like entities but a parasite can always be a soul housed within your body as well.

@Zoken44 Thank you! Glad you enjoyed it!

@LeviRocks Very true, one of the bigger inspirations for its design. Glad you enjoyed it!

@Apollo0624 Oh if only any of us could have the ear of Chris Perkins or Mike Mearls for but a moment to make a homebrew a reality, I would die happy. Thank you for the support!

Absolutely Nice !

Is there any rationale for the "advantage on concentration checks with your warlock spells" ability? Given how few spell slots warlocks get that does seem awfully strong, especially for something they get at 1st level. I also don't see how it's on-theme for a Venom analogue (Marvel symbiotes are not known for their mental stability). Compare it to the Thorn Whip ability which makes perfect sense even if it is a bit redundant given that Grasp of Hadar exists.

Perhaps as an alternative the symbiote should provide an AC of 13+DEX? This would be almost the same set of 1st level powers a Draconic Bloodline sorceror gets: armor, extra HP, and some ribbons. Obviously the flavor is very different but there would be precedent for the power set.

@Yhulths Thank you! Glad you enjoyed it!

@MrMel Mental fortitude no, but concentration checks are based on ones Constitution and since the symbiote is the one granting you the power to cast the spells in the first place I feel it makes some sense that the patron can help the player keep their wits about and maintain concentration. And while it is modeled after Venom, it doesn't mean the patron has to be an actual parasite, they can be a fragment of a powerful beings body or soul that was forced to graft itself onto the character for survival or a second psyche, kinda like a second personality, that grew in the character's mind, but I do see your reasoning behind your statement.

As far as the AC bonus goes, I do like it and it does fits in the theme of staying alive longer, a fair amount of subclasses have that feature and while the concentration bonus is powerful, paired with the warlocks limited spell pool, I feel its a good attempt and it doesn't bleed over to other classes as it specifically mentions warlock spells so I think I'll keep it as is in my version, though that is a good alternative if a DM thinks it is too potent for their personal game. Thank you for the input!

         Counterpoint: All Warlocks get the power to cast the spells in the first place from their patron. The symbiote patron is actually getting hit when the Warlock does since it's a parasite. If anything it should be the least able of any patron type to help its Warlock with concentration checks.

         Perhaps instead that could be a custom invocation associated with Pact of the Chain. If the Warlock's familiar is nearby then it helps them with Concentration because the part of the parasite in the familiar helps stabilize the part of the parasite in the Warlock.

         I should also note that a Warlock very similar to this can already be built in RAW. Fiendish Vigor for more HP (in a different way), Grasp of Hadar to pull people in, the  Enervation spell to absorb life at range, Warcaster for advantage on concentration, maybe go Hexblade since Armor of Hexes is very similar to Uncanny Deflection/Dodge and is available at the same level. Of course changing the patron itself to a parasite is just window dressing. Imagine that your Book, Familiar or Weapon lives inside of you and oozes out of you when you summon it.

        Aside from these critiques I should say that this is one of the more balanced homebrews I've seen. I especially like Symbiotic Decay which seems to be specifically about flavor.

@MrMell That... is a rather intriguing workaround that I really like... Hm. I'll mull it over and see what would work but that is a convincing argument. Thank you for the input! And really glad you enjoyed it.