The Gladiator

The Gladiator is a demon that debuted in Doom Eternal. It serves as the boss of Sentinel Prime.

Codex Entry
"In the years following the end of the Golden Age, competing factions of Argent D'Nur turned against one another. An insidious influence has set itself upon the Sentinel people, corrupting its once sacred traditions. The coliseum, once a place of honorable combat where prisoners of the Argenta were tested for the right to fight in the front lines of their army, has since become a place of blood-sport. In violation of Sentinel law, the Priests have encaged a demon there, a merciless executioner used to enact judgement of their own decree. The Gladiator, wielding an accursed shield which entombs the tormented soul of his undying master, remains undefeated in battle, infamous for the many lives it has claimed."

Appearance
The Gladiator has a large body similar to the sketch of the classic Hell Knight by Adrian Carmack, but with the addition of some body armor. Its head resembles a classic Hell Knight, but missing one of its eyes. It wields a shield and mace for weapons, changing to a pair of maces once its shield is destroyed.

In The Art of Doom Eternal, it's stated that the Gladiator is capable of resurrecting itself as long as its shield (which contains the soul of its unidentified "master") remains intact. The body will simply re-form over time and the creature's soul return to it.

Combat Characteristics
The Gladiator hides behind its shield to shrug off attacks, and has two main attacks: a lunging slam cued by the shield's eyes flashing green and the same sound cue as the Marauder's axe swing, and a projectile fired from the shield cued by the face twitching its mouth. A headshot just as the former attack is beginning will stagger the creature and place it in a state similar to a Glory Kill: performing this will deal a set amount of damage and cause the Gladiator to drop health.

Once the creature's first health bar is depleted it will lose its shield and draw a second mace. In this state it is more mobile and gains an ability to reflect damage by twirling one mace. It retains its slam attack along with a long-distance rush, more mace swings including a long-range swing, and an attack similar to the 2016 Cyberdemon where it forms a "corridor" between two energy walls and sends wavy energy lines towards the Slayer: these have "up" and "down" sections and it is necessary to anticipate their motion to determine safe spots. Once done with this attack, it will swiftly lunge into the Slayer's direction, but can be put into a stagger state with a well timed shot. These factors, along with its ability to deal massive damage, makes the Gladiator a formidable foe, and a potential roadblock during an Ultra-Nightmare run.

Trivia

 * The Gladiator's face and body bears a strong resemblance to the Barons of Hell and Hell Knights from the classic Doom games. It's a beefy faun like them, as opposed to the more ogre-like Hell Knight in the 2016 game or Doom Eternal.
 * The Gladiator's design appears to be based on the Secutor Gladiator, who fought with a heavy shield and a sword/dagger.