The Arachnomancer is based off of the Druid standard class, and as such shares the common basic skills, traits, and abilities of a traditional Druid, including harmony with Nature and with animals.
Weapon and Armor Proficiency
Arachnomancers are not known for their combat prowess, and are only afforded access to light armor, and light weapons. Popular choices include knives and daggers, staves, and basic archery. The Arachnomancer is not a melee fighter.
Arachnomancer Innate Magic
Wild Shape - the Arachnomancer gains the ability to become a human sized spider, which is in an of itself terrifying and can cause fright and feinting among those not prepared to come face to face with a 150 lb Wolf Spider or 135 lb Orb Weaver spider. The spider form remains the same from casting to casting, and the higher level the Arachnomancer is, the more capable their spider form becomes. A spider-mage can adopt a new form, or can have multiple spider forms, but this is at the cost of power. An Arachnomancer who can become three different Dire Spider Species might have greater range of spider form, but the Arachnomancer with three levels invested in Wild Shape will have a very potent Spider form.
Venom Immunity - basic ability
Form of the Swarm - the Arachnomancer can cause their body to break down into a mass of normal sized spiders equal to their weight, and become a swarm of biters. The spiders are the normal versions of their Wild Shape, and are appropriately capable. This allows them to escape harm, avoid serious injury, scare the hell out of people, infiltrate secure areas, vanish without a trace, or actually kill a very large number of people if their Wild Shape Form is particularly dangerous.
Titan Form - the Arachnomancer can become a Large Spider, reaching the size and general dimensions of a Horse, with similar stats for strength. This easily causes terror in those who see the Arachnomancer.
The Arachnomancer can gain, as a familiar, a Spider Companion. This is not a mundane spider, but is a Spider Spirit that will accompany the Arachnomancer in their travels. It appears as a spider of the arachnomancer's chosen species, but the size of a cat. It can communicate telepathically with the spider-mage, and can grant them the ability to cast magic spells.
Summon Spiders - the spell levels as the normal Summon Monster spell but only summons spiders, starting with spiders the size of dogs. These spiders are large, aggressive, and do not automatically match the summoner's chosen spider species.
Summon the Great Spider - the summoner calls forth an avatar of the Spider Spirit, which manifests as a conglomeration of species, radiates terror and fear, and is highly dangerous. It has the abilities of all spiders, and is wise and patient.
Curse of Spiders - as per the movie The Kingdom of the Spider, this spell causes a location to become overrun with spiders. These spiders will devour any prey they can, and will work together to cocoon and consume even livestock prey.
Summon the Spider-Man - The Spide-Man is a boogeyman, a creature that is only vaguely shaped like a humanoid, but it is composed of thousands of spiders, of all different sizes, that are it's flesh. It can dissolve into a swarm and reform elsewhere. The Spider Man is used as a spy, messenger, servant, and assassin, as needed.
Friend of Foe?
Arachnomancers tend to be of either a lawful or neutral alignment. They lack the empathy to be Good or the contempt to be Evil, and few are even remotely chaotic. Those that do gravitate towards evil are builders, master spies, keepers of secrets, and will infest their home location to create 'Mirkwoods', regions overrun with spiders and shadow. Those that lean towards good are patient and calculating, assassins to hunt out evil and chaos to consume it.