Poisonous fangs, toxic solutions, foul poisons. The art of an Arachnid, able to harness the deadly venoms throughout the world to their fullest. A sprinkling here, a drop there, poisons are an Arachnid's most useful tool. Arachnids inherit knowledge over generations of their ancient order, inspired by the efficiency of spiders. New students to the Arachnid archetype are trained in alchemy and poisons from a young age, and grow able to resist it's affects, while mentors and masters alike spend their days in apothcaries creating a thousand vile poisons in a single lifespan.

Poisonous Fang

3rd-level Arachnid feature

Your knowledge of poisons is a boon for you in combat. Whenever you may preform a sneak attack, you may choose to instead inflict a deadly poison. The creature is inflicted with a deadly poison, and must make a Constitution saving throw, DC 8 + your proficiency bonus + your dexterity modifier. On a failure, they take 2d4 poison damage, and become poisoned for a number of rounds equal to your intelligence modifier (minimum of 1). On a success, they take half the poison damage, and are not poisoned

You may preform this ability, a number of times equal to your proficiency bonus per short rest.

At 7th level, it increases to 4d4 poison damage, at 11th level, to 6d4 poison damage at 15th level, to 8d4 poison damage and at 19th level to 10d4 poison damage.

Careful Dosage

3rd-level Arachnid feature

You know how to apply your poisons in an effective manner. You may apply a single dose of poison to two weapons, or six pieces of ammunition. Using poisons other then basic poison, the poison damage is halved on both weapons, or on all pieces of ammunition.

 Additionally, injested and inhaled poisons ignore poison resistance.

Specialized Toxin

9th-level Arachnid feature

Your alchemical abilities allow you to create a unique poison to use. You may mix together two different poisons that are able to be applied in the same way. For example, you may combine two injury poisons, but cannot combine an ingested and a inhaled poison. You may choose to combine a single effect of being poisoned, and the damage from another. You may choose to combine the DCs of the poisons then halve them (rounding down) or you may make the DC equal to 10 + your Intelligence modifier. This process takes 8 hours.

Once you have created a specialized toxin, you may produce it for the combined cost of the two mixed poisons reduced by 10 times your rogue level (Minimum of 100), and create a single dose over the course of an hour.

You may only know how to make one specialized toxin.

Black Widow

13th-level Arachnid feature

Whenever you deal a critical hit, and use the Poisonous Fang feature, you may increase the additional damage dice to d8s. You may do this twice per long rest.

Additonally, you gain resistance to poison damage, and advantage on saving throws against being poisoned. Injury poisons also ignore resistance.

Death's Kiss

17th-level Arachnid feature

Your mastery of poison allows you to create a incredibly unique and deadly toxin, Death's Kiss. Upon creation, you may choose whether the poison is contact, ingested, inhaled, or injury. Death's Kiss has a DC of 10 + your proficency bonus + your intelligence modifier. It ignores resistance and immunity to poison damage, and deals 30d6 poison damage, half that on a successful saving throw. It takes one week to manufacture one dose, and 10,000gp.

You may only have one dose at a time. All poisons ignore resistance, and treat immunity as resistance.