Spellcasters that sought to devote themselves wholly to the Spider Queen walk the dark path of the arachnomancer. By offering up body and soul to Lolth, they gain tremendous power and a supernatural connection to the ancient spiders of the Demonweb Pits, channeling magic from that dread place. Most arachnomancers are male, as male drow cannot become priestess', but through trials given by the Spider Queen, they have earned her favor or the favor of one of her more powerful subordinates (such as a Yochlol). Most Arachnomancers are extremely loyal to Lolth but if you're player doesn't want to be bound to the Spider Queen then Vhaeraun, Selvetarm and other spider related beings could work as the Warlocks patron. 

Expanded Spell List

The Arachnomancer lets you choose from an expanded list of spells when you learn a warlock spell. The following spells are added to the warlock spell list for you.

Arachnomancer Expanded Spells

Spell Level	Spells
1st	faerie fire, jump
2nd	acid arrow, web
3rd	conjure animals(spiders only), stinking cloud
4th	phantasmal killer, giant insect
5th	insect plague, contagion

Spider Queens Favor

Starting at 1st level, you gain the ability to transform into Lolth's most beloved servitor, a giant spider. As a bonus action you turn into a spider. You gain a +2 bonus to AC. Your size becomes large. You gain 3 temporary hit-points per warlock level. You gain a climb speed equal to your normal speed. you can climb difficult surfaces, such as along a ceiling and webs, without needing to make a check. You gain a bite attack . You are proficient with this attack and it inflicts poison damage based on your level. You can use your choice of Strength or Dexterity to modify your bite attack's attack and damage rolls. While in this form, you can choose if your equipment merges into your new form or is worn by it, you still retain your ability to speak and you lack hands and cannot do anything that normally requires them. However, when you transform, you can choose a number of spells equal to your proficiency bonus to use in this form. You ignore any material components needed for these chosen spells

This form lasts for a number of hours equal to your proficiency bonus + your warlock level but can be ended early if you use a bonus action to dismiss it or are knocked unconscious

At 10th level when you use this bonus action, you can choose to become large or huge and can choose to be gargantuan at 14th.

Once you use this ability, you cannot use it again until you complete a short or long rest.

LEVEL	SPIDER FORM BITE DAMAGE
1st-4th	1d10 poison
5th-10th	2d10 Poison
11th-16th	3d10 Poison
17th+	4d10 Poison

Silken gift

Additionally at 1st level, if you do not have it already, gain proficiency with the poisoners kit and weavers tools

Poisoned Beauty

At 6th level, you gain the ability to use a shard of Lolth's alien beauty, causing other creatures to become enraptured by your appearance. As a bonus action, pick a creature you can see within 60ft of you. That creature must succeed on a charisma saving throw or become charmed by you. While charmed, it also grants vulnerability to poison damage (if the charmed creature has resistance to poison damage, it is overwritten by this. If the charmed creature has immunity, the immunity and vulnerability cancel each other out). This benefit last for one minute or until you or your allies damage the creature. Once you use this ability, you cannot use it again until you complete a short or long rest.

Demonweb Cloak

At 10th level, Lolth extends her protection to you from the demonweb. You gain immunity to poison damage and while in your spider form gain resistance to bludgeoning, piercing, and slashing damage from non-magical sources

Kiss of the Spider Queen

Starting at 14th level, you can channel Lolth's malevolence into a deadly kiss. As an action, you touch a creature within your reach. The target takes 12d10 poison or psychic damage (your choice), or half as much on a successful constitution save. A creature charmed by you has disadvantage on its saving throw. After you use this ability, you cannot cast it again until you complete a long rest.