Disclaimer: This is not play tested at all so this might be absolutely broken or incredibly underwhelming. I just liked the theme and the idea of a subclass that can be build as a caster or melee brawler.

Your Patron is a powerful being that watches over the fallen spirits and guides them on their path through the Ethereal Plane to the afterlife. You are able to gather lost souls and use their knowledge and skills from their former life to aid your cause. In exchange you connect them to your patron where they can make a deal to reach their destination safely. Depending on your Patron the destination is either the homeplane of the deity they worshiped in life or the one of the Nine Hells where souls are a sought after currency.

Expanded Spell List

The Spirit Guide 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.

Spirit Guide Expanded Spells

Optional: To publish you can only use spells from the Basic Rules. 

* replaced with Ice Knife, ** replaced with Healing Spirit

Spirit Vessel

Your patron gifts you a magical vessel that grants you the ability to gather lost souls from the ethereal plane. The vessel is a Tiny object, and you can use it as a spellcasting focus for your warlock spells. The object can be a lantern,jar,crystal, bone charm or any other object you see fit.

While you carry your vessel in one hand you can call upon the service of the souls stored in your vessel to aid you. Once per turn you can release a spirit to use one of the following options.

At level 5,10,15 you get more choices.

You can use this feature a number of times equal to 1 + your proficiency bonus. During a long rest you can reach out to the ethereal plane to gather new lost souls to guide them on their path to regain all charges.

Disruptive Spirit

Disruptive Spirit - Until the start of your next turn a creature within 30 feet of you that you can see must roll a d4 and subtract the number rolled from attacks and saving throws. 

Draining Spirit

Draining Spirit - Whenever you deal necrotic or cold damage with a spell you can send out a draining spirit. You regain hit points equal to half the amount of necrotic or cold damage dealt by one of the damage rolls.

Forceful Spirit

Forceful Spirit - Until the end of the next turn the first time you deal damage with a spell or an attack you can add necrotic or cold damage equal to 2+ your Warlock level to one of the damage rolls.

Frightening Spirit

Frightening Spirit - When you hit a creature with an attack or spell attack against a target within 30 feet you can send out a Frightening Spirit. The creature has to make a Charisma saving throw. On a failed save the target has disadvantage with attacks against you can not move closer to you until the end of your next turn. A creature immune to the frightened condition is unaffected. 

Guiding Spirit

Guiding Spirit - As an action you can call forth a guiding spirit. You can roll the next Arcana, Religion, Investigation,History or Survival check within 1 minute with advantage.

Haunting Spirit

Haunting Spirit - You send out a haunting spirit at creature within 30 feet of you that you can see. The creature has to make a Wisdom saving throw. On a failed save the target takes 1d8 + Charisma Modifier necrotic or cold damage and it has to spend its reaction immediately to move in a random direction. On a success the creature takes half damage.

The damage increases by 1d8 when you reach 5th level (2d8), 11th level (3d8), and 17th level (4d8). 

Protective Spirit

Protective Spirit - When a creature within 30 feet that you can see becomes the target of an attack you can use your reaction to send out a protective spirit. The creature can add your Charisma modifier to their Armor class until the start of your next turn.

Vengeful Spirit

Vengeful Spirit - When a creature within 5 feet that you can see misses you with an attack you can send out a vengeful spirit to retaliate. Make a melee weapon attack against the creature. On a hit you deal additional 1d8 necrotic or cold damage. The bonus damage is increased to 2d8 at level 11.

Warrior of the Fallen

You gain Proficiency in with Medium Armor and Martial Weapons as well as one of the following skills: Arcana, Religion, Investigation,  History or Survival.

Arcana

You gain Proficiency in Arcana.

History

You gain proficiency in History.

Investigation

You gain proficiency in Investigation.

Religion

You gain Proficiency in Religion.

Survival

You gain proficiency in Survival.

Awakened Vessel

As a bonus action you can spend 1 charge of your spirit vessel and awaken the souls inside. For the next 10 minutes you gain one of the following benefits:

• You gain +1 to your AC and when you take the Attack action you can attack twice. You can use your Charisma modifier for attacks and damage rolls instead of Strength or Dexterity.
• You can add your Charisma modifier to Constitution saving throws to maintain concentration on a spell and whenever you roll a ranged spell attack you can change the damage type to necrotic or cold.
• You can see invisible creatures and objects within 15 feet. Additionally you can choose the vessel to shed bright light in a 15foot radius and dim light for an additional 15 feet.

Spirit Siphon:
While the vessel is awakened you can gather the spirits of fallen nearby foes. When a humanoid creature within 60 feet of you dies, you can use your reaction to siphon it's spirit and regain one charge of your spirit vessel. You can use this feature twice per long rest.

Spirit Guardian

You learn the Reincarnate spell. To cast this spell you have to expend one charge of your spirit vessel but the material costs of the spell are reduced by half.

Whenever you finish short or long rest you gain resistance to necrotic or cold damage until you choose a different one with this feature.

Cold

You gain resistance to cold damage.

Necrotic

You gain resistance to necrotic damage.

Walking in between

Your connection to the spirits contained in your vessel grows even stronger. At the start of your turn when your vessel is in it's awakened state you can change to a different benefit.

As an action you and every willing create within 15 feet of your spirit vessel can step into the borders of the Ethereal Plane as under the effect of the etherealness spell for 1 hour.
A creature can use an action to end the effect at any time. When a creature returns to the plane it originated from in the spot it currently occupies. If it occupies the same spot as a solid object or creature when this happens, it is immediately shunted to the nearest unoccupied space that it can occupy and takes force damage equal to twice the number of feet it is moved.

Whenever you spend at least 1 minute in the Ethereal Plane you can roll a d4 and regain charges for your spirit vessel equal to the number rolled. You can use this feature once per long rest.