101st Legion

The 101st Legion of the Alliance of the Dawn are a group of surviving soldiers from the Solar-Break. Members of the 101st volunteered to undergo the process to become vampires to better aid in the battle. They are some of the only people to have first hand knowledge of the battle, and their defeat at the jaws of the God-Dragon Rhothaxis.
The 101st Legion are ancient now, long-undead vampires turned by Severa Willow thousands of years ago. They stand a foot taller than any normal man, fight with heavy rifles over three feet long, and have inspired legends and night-terrors for all who live in the true north. Their armour is tougher than steel, their bones wrought of iron, their minds drained of fear. They know only one purpose: War. And war they shall have, the brothers of the 101st Legion.
Five hundred men belong to the 101st, bound by blood-magic to serve Severa Willow's command.
Their armour is built to fully encapsulate it's wearer, designed from the original Armour of Invulnerability worn by Kaiser Arkus Nephiron I tens of thousands of years ago. It lacks the original's true protection, but a true improvement on the design. A glass-visored helmet sits atop the suit, providing the user with real-time battlefield analysis and vastly improved perception.
It is a commonly held belief among modern scholars that the 101st Legion was the first to undergo Severa Willow's procedure at Site-19. Several recordings survive of the speech she gave prior to the creation of the 101st:

My dear warriors, I stand before you today as a being of great strength and power, a vampire. It is a path I have travelled, one that came with great sacrifice. But I stand here before you, immortal and unrivalled by any foe. 
I know that the enemy we face is formidable, and the stakes could not be higher. But I have faith that together, we can turn the tide of this coming war in our favour.
That is why I offer you the same gift that I have received. The gift of immortality, the power of the vampire. It will not be an easy journey, but I have seen firsthand the strength and skill that each and every one of you possess. You are warriors like no other, and I know that with you by my side, we can conquer any obstacle. So I ask you, my legions of Iron and Steel, will you join me on this journey?
Forged not in life, but in the fires of death shall you harden into my instruments of violence.
My armour shall be your coffin of steel, fuelled in death by the rage of this life. This iron-sarcophagus will guard you, unbreaking and unwavering as the spirit of the bones it entombs.
Wielding only the keenest of blades, you, my soldiers, shall piece the toughest of shields. Armed with tools so deadly, naught but even the gods shall fear them.
You shall be the guardians of the living. Men who die, so that others may live. You shall share my blood. You will know no rest. You will feel no joy. And you will know no fear.

Several accounts from stewards and other staff of Site-19 report several previously unknown aspects of the procedure and surrounding events, contrary to the eyewitness accounts given by surviving members of the legion.
Notably, most accounts include the forcible execution and subsequent resurrection of at least seven volunteers. Further damning testimony includes a recovered analysis of the procedure which reports a failure rate of 82% for the initial run for the 101st Legion. Further legions seem to have an even higher failure rate, potentially due to even lower entry requirements.
In game, they are made up of Vampiric Legionnaires.

ChatGPT

Vampiric Legionnaire CR: 6

Medium undead, any

Armor Class: 16

Hit Points: 75 (10d8+30) 10d8+30

Speed: 30 ft

STR

16 +3

DEX

14 +2

CON

16 +3

INT

10 +0

WIS

12 +1

CHA

16 +3

Skills: Regeneration. The Vampiric Legionnaire regains 10 hit points at the start of its turn if it has at least 1 hit point.
Spider Climb. The Vampiric Legionnaire can climb difficult surfaces, including upside down on ceilings, without needing to make an ability check.
Vampire Weaknesses. The Vampiric Legionnaire has the following flaws:

• Forbiddance. The Vampiric Legionnaire can't enter a residence without an invitation from one of the occupants.
• Harmed by Running Water. The vampire knight takes 20 acid damage when it ends its turn in running water.
• Stake to the Heart. If a piercing weapon made of wood is driven into the vampire knight's heart while it is incapacitated in its resting place, the vampire knight is paralyzed until the stake is removed.
• Sunlight Hypersensitivity. The vampire knight takes 20 radiant damage when it starts its turn in sunlight. While in sunlight, it has disadvantage on attack rolls and ability checks.

Damage Immunities: necrotic, poison

Condition Immunities: charmed, exhaustion, frightened, paralyzed, petrified, poisoned

Senses: darkvision 60 ft., passive Perception 11

Languages: Common

Challenge Rating: 6

Actions

Multiattack. The Vampiric Legionnaire makes two longsword attacks. Longsword. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 7 (1d8 + 3) slashing damage plus 7 (2d6) necrotic damage.
Energy Blast. Ranged Weapon Attack: +5 to hit, range 30 ft., one target. Hit: 10 (3d6) force damage.
Energy Drain (1/day). The Vampiric Legionnaire targets one creature within 5 feet of it. The target must succeed on a DC 14 Constitution saving throw or take 14 (4d6) necrotic damage and have its hit point maximum reduced by an amount equal to the damage taken. This reduction lasts until the target finishes a long rest. The target dies if this effect reduces its hit point maximum to 0.

The Vampiric Legionnaire is a formidable undead warrior, standing tall and imposing in its gleaming plate armor and wielding a longsword with deadly precision. Its eyes burn with a sinister, otherworldly light, and its skin is pale and cold to the touch. Despite its undead nature, the Vampiric Legionnaire moves with a grace and agility that belies its undead state. When it speaks, its voice is deep and commanding, echoing with the power of the abyss. As it prepares to attack, the vampire knight charges its longsword with a crackling energy, ready to unleash a powerful blast at its foes.

Taken from Dead Legions by Pharon Delos.