Gladiator: Path of the Raging Fiend

Barbarians are known to give into rage.  To embrace the beast inside, and use it to vanquish their foes with a primal fury unknown to most civilized individuals.  For the Gladiators who follow the Path of the Raging Fiend, they go even further.  Dig deeper than anger, beyond rage, and touch their inner darkness.  Given unnatural powers, they forever must do battle with themselves to maintain control.  Rage and anger meet a perverse need to do harm, while the Barbarian attempts to control it.   Many times, the Gladiator gives in completely, becoming little more than a raging beast who revels in the pain and misery of all around them.  Others maintain control, as much as can be expected.  As one who follows the Path of the Raging Fiend, you must be ever mindful of yourself... or perhaps you embrace your darkside completely.

Darkside

Starting when you take this subclass, you've learned to embrace your inner darkness.  This can manifest in many ways.  Glowing eyes, a dark aura, your shadow turning into something sinister, or even being wreathed in a demonic shroud.  When you rage, it manifests fully, but small signs are always there.   You gain proficiency in Intimidation.

In addition, while you are raging, you may use a bonus action to make a weapon attack.  On a success, you deal your weapon damage, although it is necrotic instead of your weapon's normal damage.  You may use this a number of times equal to your Proficiency Bonus, regaining all uses after a long rest.

Darkness Unbound

Starting at level 6, your eyes become accustomed to the dark.  You gain Darkvision at 60 feet, or 120 feet if you had Darkvision already.  In addition, when entering a rage, you gain THPs equal to half your level plus your Proficiency Bonus.   These hit points vanish when you end your rage, but are fully replenished if you use your Darkside as a bonus action to attack.

You also gain Thaumaturgy as a cantrip.  When you enter a rage, you may use Thaumaturgy as a free action.

Lord of Savagery

Starting at level 10, while raging you gain resistance to Fire, Cold, Poison, and Lightning damage.  

In addition, due to giving into your more feral and animal like nature, you gain the ability to cast Speak with Animals at will. 

Evil Never Dies

Starting at level 14, you learn to speak Abyssal or Infernal.  In addition, when you reduce a hostile creature to 0 hit points or lower, you may regain a use of Darkside.  You may only use this once per short or long rest.

If you choose Abyssal, you gain advantage on Charisma checks with Demons whose CR is equal or lower than your current level.  Demons become Unfriendly to you, but are not outright hostile.   

If you choose Infernal, you gain advantage on Charisma checks dealing with Devils whose CR is equal to or lower than your current level.  Devils become Unfriendly to you, but are not outright hostile.  

You may choose instead Infernal or Abyssal, choosing the middle path of evil.   Choosing this, you gain advantage on Charisma checks with Yugoloths whose CR is equal or lower than your current level.  Yugoloths become Unfriendly to you, but are not outright hostile.