The Heralds of Unicron are Transformers who have entered into service for Unicron. They have appeared as antagonists all throughout the Transformers franchise.
The Transformers: The Movie
Coming across a group of injured Decepticons that included Megatron, Unicron rebuilt them as his followers, reformatting Megatron into Galvatron, Thundercracker into Scourge, either Skywarp or Bombshell into Cyclonus and the rest into the Sweeps.
Unicron then sent his heralds to retake control of the Decepticons and destroy the Matrix of Leadership. However, Unicron was later destroyed by the Autobot Hot Rod via the Matrix of Leadership.
However, Galvatron, Cyclonus and Scourge survived and later returned to lead the Decepticons, however they acted independent from Unicron and did not carry out his will.
The Transformers: Season 3
Unicron later made a deal with the ghost of Starscream, offering to revive the Decepticon in exchange for him giving Unicron a new body. However, Starscream backstabbed Unicron right after the Chaos Bringer resurrected him, leaving Unicron to be destroyed by the Autobots via Energon bombs.
Beast Wars II
Unicron at some point created the trio of immensely powerful Transformers known as the Blendtrons. Some time after Unicron's destruction, the Blendtrons ambushed the Maximals and Predacons and stole all their Angolmois, which was actually the life force of Unicron. They then released the Angolmois into the deceased Galvatron, reviving Unicron in the Predacon leader's body.
However, right after his resurrection, Unicron absorbed the Blendtrons into himself to gain their energy.
Unicron sent his herald Sideways to sow hatred and discord between the Autobots and Decepticons. Sideways initially posed as an Autobot, but later switched over to the Decepticons. Sideways attempted to usurp control of the Decepticons by turning Megatron and Starscream against each other. Eventually though, the Decepticons discovered what Sideways was up to. After Sideways was discovered communicating with an unseen entity, Sideways was forced to flee the Decepticons by entering into Trans-phase mode.
Sideways later made several more appearances to convince both the Autobots and the Decepticons to gather the weapons needed to form the Hydra Cannon, the Star Saber, Requiem Blaster and Skyboom Shield. He also recruited Thrust as another herald of Unicron. Eventually, fearing that his alignment with Sideways would be discovered, Thrust stole the Requiem Blaster and Skyboom Shield to bring to Unicron. Starscream noticed this and pursued Thrust. Sideways attempted to recruit Starscream as another minion of Unicron, but Starscream refused. Sideways then attacked Starscream with strange tendrils and amoeba like organisms, driving away the Decepticon and allowing him and Thrust to escape.
With the three weapons placed inside Unicron's core, Sideways told thrust to leave. Thrust was confused, as he believed he was Unicron's right-hand man. Thrust attempted to become Unicron's personal tactician, but the Chaos Bringer took no notice of the former Decepticon and Thrust was crushed to death when Unicron transformed.
After Optimus Prime and Galvatron made it to Unicron's core, Sideways appeared to them and revealed that he was nothing more than a "shell" for Unicron. He was later killed by Optimus with the Requiem Blaster. However, right before passing away, Sideways claimed he'd have his revenge.
Sideways returned in Transformers: Cybertron, but rather than continue serving Unicron, Sideways opted to carry out his own agenda.