Oath of Limbo

You are permanently in a form between life and death. This feature lets you connect with the dead while still living among mortal men. 

Tenets:

Vengeance for the Dead: You have formed connections with various dead creatures. When a creature is unjustly affected, you seek vengeance for them. Even when they die, you continue the mission because you still hold a connection with them.

Search of Knowledge: When the dead can share their knowledge, nothing should be lost or forgotten. The thought of information disappearing infuriates you and, you seek to protect it.

Sympathy for the Dead: You are very close to many dead creatures. You sympathize with many of them and will try to help with any dying wishes that were never fulfilled.

Channel Divinity

When you take this oath at 3rd level, you gain the following two Channel Divinity options.

Knowledge of the Dead. As an action you call upon the dead to gather information. When you do so, you learn about any vulnerabilities, resistances, and immunities for a number of creatures equal to your charisma modifier of your choice.

Necrotic Absorpstion. As a reaction, you channel undead energy to heal you. When Necrotic, Poison, or Acid damage is done to you, you can use your reaction to regain hit points equal to half the damage rounded down.

Oath Spells

You gain oath spells at the paladin levels listed.

Oath of Devotion Spells

Aura of Death

Starting at 7th level, all creatures you choose within10 feet of you have disadvantage on saving throws against disease, poison, and necrotic spells. If a creature in the area would regain hit points, it must make a Constitution saving throw. On a failure, the creature regains no hit points. If the saving throw succeeds, the number of hit points regained is instead halved. This aura has no effect on undead or constructs.

At 18th level, the range increases to 30ft.

Grave Messenger

You can cast speak with dead at will as long as you have the material components with you. 

Fearful Form

At 20th level, as an action, you can transform into a walking corpse. For the next minute you gain the following benefits.
You can walk through solid objects if you choose to. You can treat this as normal terrain and if you end your turn inside of an object, you take 1d10 force damage and are pushed out in a direction determined by your DM. You gain immunity to Necrotic and Poison damage as well as the Poisoned. You have advantage on all weapon and spell attacks against living creatures. This feature can be used once and recharges when you take a long rest.