I'm still poking at this one... since Aranea are pretty much a PC race in the SC:
Aranea 5E conversion
by argentmantleAbility Score Increase: Your Dexterity score increases by 1, and either your Dexterity or your Intelligence increases by 1 (your choice).
Darkvision: You can see in dim light within 60 feet of you as if it were bright light, and in darkness as if it were dim light. You can't discern color in darkness, only shades of gray.
Change Form: As an action, you can transform into between your forms. The arachnoid can take on demi-spider form, the humanoid can switch to demi-spider, or the demi-spider can switch to either form. This means that to transform from humanoid to arachnoid requires two actions be spent.
When an Aranea character is created, they choose a form of any medium humanoid race: any component of appearance including your height, weight, facial features, the sound of your voice, coloration, hair length, sex, and any other distinguishing characteristics. Once you have chosen this form, it becomes the permanent humanoid form for the Aranea.
It is important to note that most gear does not transform with the character from demi-spider to arachnoid form, leaving it discarded on the ground.
Aranea who are knocked unconscious stay in their current form. If slain while in demi-spider form, the character has a 50% chance to revert to their arachnoid or their humanoid form.
Alternate Persona: You gain proficiency with one tool of your choice. While you are in the form of this persona, you fill that role without raising any suspicions of your dual identity. If necessary, an Aranea gains advantage on all Charisma rolls to allay suspicions.
Arachnoid Build: When in Arachnoid form, you count as one size larger when determining your carrying capacity, the weight you can push or drag, and determining size for grappling.
Spider Climb: When in Arachnoid or Demi-Spider form, Aranea can climb any surface without making ability checks.
Bite: When in Arachnoid or Demi-Spider form, your fanged maw is a natural weapon, which you can use to make unarmed strikes. If you hit with it, you deal piercing damage equal to 1d6 + your Strength modifier, instead of the bludgeoning damage normal for an unarmed strike.
Webbing: When in Arachnoid or Demi-Spider form, you can extrude webbing from your spinnerets, this amounts to about 50 feet of spider silk per day. As an attack, you can project this at an opponent by making a ranged attack. On a hit, the target must make a Dexterity save or become restrained unless they pass a Dexterity save. A character may do this a number of times equal their Constitution modifier (minimum of 1) before needing to recharge this ability during a long rest.
Poisonous Bite: At third level your bite in Arachnoid and Demi-Spider form becomes is poisonous. When you make a successful attack with your bite and use your poisonous bite, you may force the creature to make a Constitution saving throw or become poisoned until the end of your next turn. The DC for this saving throw is equal to 10 + your proficiency bonus + your Constitution modifier. Once you use this feature you must finish a short or long rest before using it again.
Notes
Much of the Aranea’s shapechanging abilities are inspired from the Changeling presented in Wayfinder’s Guide to Eberron. However, they have an exceptional number of abilities, and a really think that DMs should consider whether to allow them in your campaign, review the material provided here, and make any changes they see fit.
I'm actually for striking the Poisonous Bite ability immediately from the class and making it a feat instead of granted ability.