Kirathel, AKA Darthvega98, is an old user from the OG TNC, who has both too much time on his hands and not enough. Edit
Rahzahkea, a Toa of Psionics from a forgotten age. Blessed with a sword of eldritch power and a powerful mask, his zeal for justice and idealism have long since been dulled, replaced with a burgeoning hatred of the monsters that have attacked New Atero.
It's going to get him hurt someday.
Sylah, a former Vo-Matoran turned mutant abomination turned technically-still-not-a-Toa with powers over Lightning and Shadow (also Darkspace but we don't talk about that here). Though still burdened by the past and the monsters within, she has moved on much better than Rahzahkea, and serves in the Vanguard alongside him, defending their newfound home.
Kirathel, a golden, Makuta-like being codified as a Demiurge, who forsook her place to destroy her world's cosmic order and replace it with perfect madness. Light Is Not Good, and she is not good either. Perhaps once, but....no more.
Shard, a late Toa of Iron. Once a hero, turned a dark, and cruel mirror of himself, he fought alongside the Vanguard with the sole goal of inflicting pain and torment on the demons that attacked New Atero. He spurned both 'Kea and Sylah, his former friends, until his very end.
He is not missed.
The Heralds, broken, malformed clones of people Kirathel once knew, created to "aid" the Vanguard;
- Inferna, a Rahkshi-Girl-Stereotype that somehow parasitized her own pain to gain more power, with powers over fire. Oddly enough she was the most helpful to the heroes. She's dead now.
- "Not-Kiyara", a ghostlike thing that could teleport and turn intangible. He literally never did anything besides fly around and cackle evilly. He's also dead now.
- Lumos, a very, very sad Hau-bearing Toa with the amazingly unimpressive power of being a living Mask of Illumination that could also fly. He was nice, but didn't really do anything. He's also dead now.
- Selarial, aka Not-Eclipse, a fragile, golden clone of Kirathel with even less reason to be evil, and the (ironic) ability to project a weakness field (that is also amazingly unhelpful because AOE attacks). She offered to help the heroes, but given that she did it after almost turning one of them into a Marendar-rip-off, she was rebuffed, and was real broken up about it. Though the Vanguard confronted her at the Tower, she eventually agreed to let them pass, and a giant clone of Teridax cast her into the fire (don't ask).
Is she dead?
Yeah she's probably dead by now.
The Wrathbringer, El-Arachis, the Destroyer of All, the Sister-Brother of the Nail-Bearer. A monster from beyond time, a lonely, cruel god with no interest in chaos or order. Unlike most of the other villains from the series, it seems to have little interest in conquering all things, dominating them, or torturing the heroes for personal enjoyment. Rather, it intends merely for Creation to cease. For Order to cease. For Chaos to cease. And there shall be nothing, but in that nothing it will reclaim its lost kin. And it will not be alone.
Kiyara, an Imbiber "scientist" who served Kirathel out of a perverted affection. A "monstrum ex nihlo", with no excuse to justify his cruelty, just a penchant for making monsters, all in service of his only true "love". A madman who needed to be put down.
A pity they failed.