They were a trio of bounty hunters who possess no honor and have killed people just for a single bounty. They were hired by General Morando to kill Prince Krel and Princess Aja and procure King Fialkov and Queen Coranda's cores.
It's unknown how the Brotherhood was formed.
At one point, hundreds of years ago, the Zeron Brotherhood attempted to steal Gaylen's Core from the guard of House Tarron for unknown purposes. However, Fialkov and Coranda managed to stop them and handed the core to the Trollhunter named Kanjigar the Courageous, who kept it safe within a cavern he made called "the Deep", where anyone who enters it faces their greatest fear.
It's also revealed that after General Morando was exiled from Akiridion-5 for high treason, he hired the Zeron Brotherhood to cause a massive bombing, thus killing Varvatos Vex's family and hundreds more Akiridions. Despite Varvatos begging King Fialkov to interfere with it, the latter refused as he was more interested in increasing the defenses of the planet. Following this, Morando ordered Varvatos to lower the shields while promising no one would be hurt upon his return, which he did upon being blinded by his anger.
3Below (Part One)
Following his successful takeover of Akiridion-5, Morando hires the Zeron Brotherhood to be part of his bounty hunters across the galaxy to murder Aja and Krel Tarron and bring life cores of their parents to Morando. They began their search at the Intergalactic Way Station, where they find out from a Foo-Foo the royals' whereabouts, and slaughter everyone listening to them. When they travel to Earth, they capture a Skelteg and use it to discover their location further.
The Brotherhood attempt to nab the royal children during their driver's ed class. Being as resourceful as they are, Krel upgrades their car to make it go faster, but when they are caught in their retrieval ray, Aja uses her serrator to destroy their ship's stabilizer. The children recover a part of their ship and discover the identities of their attackers.
Soon, Varvatos decides to confront them himself and gains the upper hand at first, but they prove to be too much for him. However, Aja arrives and helps him fight them off, while Krel sabotages their spaceship so they couldn't follow them.
After finding out that they cannot break through the Mothership's defensive barrier, the Zeron Brotherhood decide to wait for the royals to be vulnerable enough to be attacked by them, which happens when Aja and Krel go to a bonfire, where Aja has her first date with Steve Palchuk. As Aja and Krel are being cornered by the Zeron Brotherhood, Varvatos arrives and kills Zeron Beta. Following this, Alpha and Omega express grief at his death.
Alpha and Omega eventually manage to capture Aja, Krel, and Steve inside their spaceship. During this time, Alpha threatens Varvatos that if he doesn't surrender the King and Queen's cores, then he will kill Aja and Krel. Because of this, Varvatos decides to trick them with fake royal cores that are actually bombs. When Alpha discovers that he was lied to, the bombs explode, and the spaceship plummets from the sky, and Steve finds out that Aja is an extraterrestrial. When they are both attacked by Omega, the spaceship crushes the latter.
While Alpha mourns Omega's death, the first Omen arrives to Earth, and Morando uses it to tell Alpha that, since he and his brotherhood have failed in their task, he should submit to the Omen if he wanted to live. Alpha ultimately, yet fearfully, agrees to this.
Eventually, after the dark witch, Morgana, creates the Eternal Night and sends Gunmar and the Gumm-Gumms to vanquish mankind, Alpha uses this time to track Aja and Krel to the Mothership and ambush them, where he harms Zadra. While the Omen corrupts Mother's programming to make her leave Earth, Alpha keeps the royal children busy, but is knocked away by Buster. However, after Varvatos makes a sacrifice to crash the Mothership so the King and Queen's cores could remain safe from Morando (all while Merlin ends the Eternal Night), Alpha kidnaps Varvatos and takes him to the bounty hunters' outpost on the Moon, where he keeps him in confinement. He plans on returning him to Akiridion-5, where he would face "high treason" from Morando.
3Below (Part Two)
After the royals rescue Varvatos from his prison, he goes on a one-on-one combat duel with Zeron Alpha outside the outpost. There, Alpha reveals that Morando sent him and his two late comrades to orchestrate the tragic bombing that killed his family, specifically so that he would be vulnerable enough to lower the shields. Following this revelation, Varvatos kills Alpha, thus avenging the deaths of his own family.