The Spiritbreaker

The powers of a Spiritbreaker are powerful and only known to beings with a strong connection to their own souls. This could be the spirit of a powerful, fallen sorcerer, or potentially a lich; while most evil liches would fit a warlock of the Undying more than the Spiritbreaker, but a demilich or a good-aligned lich may be willing to teach their warlock the ways of the soul. The spells and abilities that are granted from a Spiritbreaker are cast from the soul of the caster, and directly affect the spirit of the target in some cases, either bolstering it, withering it away, or altering its decisions.

Expanded Spell List

1st level: Heroism, False Life
2nd level: Warding Bond, Calm Emotions
3rd level: Bestow Curse, Revivify
4th level: Compulsion, Death Ward
5th level: Modify Memory, Raise Dead

Soul Touch

Your connection to souls gives you the ability to alter the decisions of those around you directly through their spirits. When a creature that you are not in active combat with is making a conscious, meaningful decision that you are aware of (up to DM discretion whether those requirements are fulfilled), you can use an action to try to force that creature to make the decision that you want it to. That creature must make a Wisdom save (DC equal to your spell save DC), and, on a failed save, that creature makes the decision that you ask or tell it to make, without knowing that you affected it, and does what it normally would do after the decision. You cannot force any creature to do anything that would cause direct harm to themselves or anyone that they are friendly towards, and said creature may question an action that it is forced to take that is out of character, but it won't go back on it. On a successful save, the creature knows that you used this ability on it. 

Once you use this ability, you can't do so again until you finish a short of long rest. 

Puppet Maneuver

Starting at 6th level, you learn how to transfer you spirit to another creature for a very short time. As an action, choose one creature within 30 feet of you. That creature must make a Wisdom saving throw (DC equal to your spell save DC). On a failure, you can force that creature to use its reaction to do one of the following:

-Make a single action, doing anything that the creature would normally be able to do using one action

-Move up to its speed

Once you use this ability, you can't do so again until you finish a long rest.

Spirit Tether

Starting at 10th level, you gain more control over you own spirit, even in times of crisis. When reduced to 0 hit points, you make death saving throws with advantage.

Master Possession

Starting at 14th level, you learn how to transfer your spirit to other beings for extended periods of time. As an action, choose the corpse of a creature within 30 feet. The creature must have a CR that is at least half your warlock level. You then transfer you soul to that creature. Your normal body simply stops and collapses, but is not immune to damage and is considered unconscious. This spell closes all mortal wounds, but it doesn't restore missing body parts. If the creature is lacking body parts or organs integral for its survival--its head, for instance--the spell automatically fails. 

While possessing the creature, you gain the speed, ability scores, skills, languages, and senses that the creature had in life, and your hit points in this form are equal to half the creature's hit point maximum in life. In addition to that, you have access to any abilities and actions that the creature had in life as long as the creature still has what is required for those abilities, and any abilities and actions that you have that your new form would reasonably be able to use. You can still cast any spell that you could normally cast, as long as the new form can still provide all required components for the spell.

If your hit points in the creatures form are reduced to 0, your spirit returns to your normal body, as long as it is still alive. If your body is reduced to 0 hit points while possessing another creature, then it still makes death saves on your turn without the aid of your Spirit Tether ability. If your body dies, your spirit stays in the possessed form until it is forced to leave. 

You can possess a creature for up to 1 hour before you are forced out of its body. Once you use this feature, you can't do so again until you finish a long rest.