Path of the Dark Side

The emotions that empower a Barbarian's rage can include anger, pain, or sorrow. Those n the Path of the Dark Side harness these negative emotions into an umbral power not unlike that of the Shadowfell. These dark knights endure as much harm as they can muster, then unleash it back onto their enemies.

Vengeance, self-loathing, or the desire to protect others are among the many reasons a Barbarian would seek this path. The Dark Side welcomes both the saviors and the destroyers.

Dark Knight

At 3rd level, you can withstand the burden of your ebon armor. 

You gain proficiency with heavy armor, and you can gain the benefits of your Rage even while wearing it. Your armor helps suppress your feelings until they're ready to be unleashed. While you are wearing heavy armor and not Raging, you can add your Rage Damage value to ability checks and saving throws against attempts that try to discern your intent, emotions, or thoughts.

Soul Eater

Also at 3rd level, you can turn your suffering into strength.

When you enter your Rage, and at the start of each of your turns you are Raging, you can choose to take 1d6 points of necrotic damage, which cannot be reduced by any means. Until the end of that turn, your weapon attacks deal extra necrotic damage equal to 1d6 + your Constitution modifier.

If you score a critical hit while this ability is active, you gain temporary hit points equal to the necrotic damage you deal. Moreover, if you are below half your hit point maximum, you can choose to maximize both the necrotic damage you take, and the necrotic damage you deal with this feature.

Shadow Wall

At 6th level, the power you pull from your emotions has gotten stronger, but so have you.

You gain resistance to necrotic damage. Additionally, you can draw harm away from others. When a creature within 15 ft. of you takes damage while you are raging, you can use your reaction to take that damage instead.

Dark Mind

At 10th level, your imposing presence aids you both in combat and in conversation. 

You can interrogate a creature alone for at least 5 minutes. At the end of the interrogation, you can make a contested Charisma (Intimidation) check against its Charisma (Deception) check. If you are successful, you can ask it up to three questions, which it must answer truthfully.

The creature has disadvantage on this check if you have damaged it in the last 24 hours, and if it is either below half its maximum hit points, or if it was knocked Unconscious within that time frame.

Once you use this feature, you cannot use it again until you finish a short or long rest.

Quietus

At 14th level, you can sacrifice yourself to end your foes.

When you hit a creature with a melee weapon attack while raging, you can expend up to half your hit dice. Roll the expended hit dice, and both you and the target take additional necrotic damage equal to the number rolled, ignoring any sort of resistance either of you may have.

Once you use this feature, you cannot use it again until you finish a long rest.