The Emotions that empower a rage can include anger, pain, or sorrow. Those on 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
3rd-level Path of the Dark Knight feature
At 3rd level, you can withstand the burden of your ebon armor. You gain proficiency with heavy armor and you gain the benefits of raging even while wearing it.
Your armor helps suppress your feelings until they're ready to be unleashed. While you are wearing heavy armor and you're not raging, you can add your Rage Damage value to ability checks and saving throws against attempts at discerning your intent, emotions, or thoughts.
Soul Eater
3rd-level Path of the Dark Knight feature
Also at 3rd level, you can turn your suffering into strength. When you enter your rage and at the start of each turn you are raging, you can choose to take an irreducible 1d6 damage. 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 Soul Edge is active, you gain temporary hit points equal to the necrotic damage dealt.
If you are below half your maximum hit points, you can choose to maximize both the damage you take and the necrotic damage you deal with this feature.
Shadow Wall
6th-level Path of the Dark Knight feature
At 6th level, you gain resistance to necrotic damage.
Additionally, you can draw harm away from others. When a creature within 15 feet of you takes damage while you are raging, you can use your reaction to take that damage instead.
Dark Mind
10th-level Path of the Dark Knight feature
At 10th level, your imposing presence aids you in both 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 it either is below half its maximum hit points or if it was knocked unconscious within that time frame.
Once you use this feature, you can't use it again until you finish a short or long rest.
Quietus
14th-level Path of the Dark Knight feature
At 14th level, you can sacrifice yourself to end your foes. When you hit a creature with a melee weapon while raging, you can expend up to half your hit die. Roll the expended hit dice and both you and the target take additional necrotic damage equal to the number rolled, ignoring resistance.
Once you use this feature, you can't use it again until you finish a long rest.