Arachnomancers are unique individuals who have an innate and magical similarity to spiders. Either bitten by an enchanted spider and gifted with these powers, cursed by Lolth, or the humanoid child of a drow drider, an arachno-mancer can gain their powers in a number of different ways. Alongside physical similarities to spiders, such as the ability to generate webbing and climb walls, this connection with a creature of the natural realm gives an arachno-mancer innate magical abilities; they're able to weave the fabric of magic, as they are unique and evolved individuals with strong connection to the magic that created them in the first place.

Arachnid Physiology

Beginning at 1st level, an arachnomancer develops the physical properties of a spider. You have a climbing speed equal to your walking speed, and can climb difficult surfaces, including upside down on ceilings, without needing to make an ability check. Additionally, you ignore the movement restrictions caused by webbing.

Friend of Spiders

Also at 1st level, you may cast the animal friendship spell, but only to target a spider, and it has disadvantage on its saving throw. Once you cast this spell in this way, you must finish a short or long rest to do so again.

Webbing

From 6th level, you gain the ability to create and control spider webs as innately as spiders do themselves. You can cast the web spell at the cost of 3 sorcery points, ignoring the verbal and material components. When cast in this way, you do not need to maintain concentration on the spell. While in contact with a web, you know the exact location of any other creature in contact with the same web.

Additionally, you can shoot concentrated webbing at creatures. For 1 sorcery point, you can make a ranged spell attack against a creature within 60 feet. On a hit, the creature is restrained until freed. The escape DC is equal to your spell save DC.

Venomous

At 14th level, your arachnid physiology extends to the venomous properties of the most dangerous of spiders. You gain resistance to poison damage and immunity to the poisoned condition. Additionally, once per short or long rest, you can excrete a natural venom that you can coat a weapon with as a bonus action. You do not have to be wielding the weapon for the venom to remain. The venom remains for up to 1 minute before drying, and the first hit made against flesh with that weapon after being coated deals an additional 8d4 poison damage. A creature hit with this venom must make a Constitution saving throw against your spell save DC or be poisoned for 8 hours.

Swarm Form

Upon reaching 18th level, you can fully embody the spidery origin that is the source of your power. Once per long rest, as an action you can disintegrate your own body into a swarm of spiders. Your spider swarm covers a diameter of 10 feet and has 50 hit points and an AC equal to your spell save DC. While in swarm form, you have a walking speed of 25 feet and a climbing speed of 25 feet. On your turn, you can make an attack against every creature within your swarm form. You add your proficiency bonus and Dexterity modifier to the attacks, which on a hit deal 2d4 piercing damage plus 4d6 poison damage. When your swarm form is at 25 hit points or less, this damage becomes 1d4 piercing damage plus 2d6 poison damage. 

While in swarm form, you can expend 2 sorcery points to regain 1d8 hit points, but cannot regain hit points in this form by any other means.

You can remain in swarm form for up to 1 hour or until you revert to your original form as a bonus action. If you are reduced to 0 hit points in swarm form, or you reach 1 hour, you revert back to your normal form with whatever hit points you had upon entering swarm form. Damage that returns you to your original form carries over if in excess of the 50 hit points of your swarm form.