Overview and origins.
The Enclave Onyx Trooper is essentially the brainchild of Special Agent CRONOS, who during downtime between missions conducted the development and creation of the Onyx Advanced Power Armor with the resources he had recovered over the years.
A lighter, stealthier variant of the Mark II Advanced Power Armor, it features dual mode servos, capable of operating in fully functional high powered and/or low noise/low power modes. It's most distinguishing and unique feature is an integrated stealth field generator developed with the help of Doctor Henry and his Mark II Stealth Boy prototype.
CRONOS also made sure to install a self-destruct sequence initiated by an on-board heartbeat sensor to mask it's inner workings to groups such as the Brotherhood of Steel and New California Republic, and to guarantee no intact suit would ever be obtained by an enemy.
As for it's overall appearance, before even beginning the project CRONOS knew he wanted it to inspire a new sense of fear in the enemy of the Onyx Troopers he would soon train.
It's lenses are tinted an eerie olive green and it's color is a deeply darkened matte black, inspired by the original state of the Mark I Advanced Power Armor in the 2240's, yet being made even darker to suit it's combat role and inspire a sense of dread in the enemy.
Fighting terror with terror.
"Terror Militantur Terrore..."
-Motto of the Onyx Troopers.”
Despite his deep hatred for the NCR and all who belong too it, CRONOS holds a grudging respect for the NCR's Rangers, purely in just how effective they have proven to be. By CRONOS' calculations, were it not for the Rangers the NCR's progress would have been far less than it is and as a result the damage inflicted to the Enclave would be far less catastrophically crippling. Thus, he was convinced a counter must be made to this...
Determining that their training is dangerously effective, 'too good', CRONOS brought the specifics of his plan of a new, elite, and secretive variant of soldier to the higher ups.
Due to CRONOS' near-flawless service record, mission success rate, the assurance that he would train the new trooper variant himself, as well as the costly failure of Enclave Sigma, the remaining Enclave leadership were convinced CRONOS' brainchild for a perfect soldier could quite possibly insure not only the Enclave's continued survival, but perhaps even give them an edge they desperately needed with a personalized counter to the guerrilla warfare tactics that has bested them.
After the DC Campaign but before his assignment to Nevada, CRONOS spent those years training exactly 35 out of 40 up and coming Enclave recruits from the ground up, killing 5 of these prospects personally for what he called 'treacherous incompetence'.
Molding them in his image and his expertise with mercilessly brutal training methods, he even went so far as to have them volunteer for the same armor grafting process he and Frank Horrigan before him underwent.
CRONOS wanted this process to insure an arguably perfect loyalty in his new legacies akin to his, but instead of just his mere unquestioning loyalty, he wanted that -in addition- to new and upgraded cybernetic technology, including ocular, cerebral, and stimulant implants designed to give the Onyx Troopers an edge so beneficial that it subliminally inspires even further loyalty...
All of this coupled along with the Enclave's already top-notch elite pre-War training method(s), they are incredibly resilient, fast acting, and eccentric soldiers in that they are given leeway as devoted but independent field operatives.
They are far from unbeatable in spite of their intended design to be, their main flaw is that there are only 35 of these trump-card shock troops, but the Onyx APA MK II also offers less protection than the original APA MK II due to it's sophisticated stealth system and the other resources being committed to the Onyx Troopers' cybernetics and grafting procedure.
The only known sighting of one of these operatives is regarded as a gossiping rumor that started in a bar to scare people because they are described as 'not sounding or looking anything like a man or woman, more like a robot monster of some kind'...
(This fanon was inspired by a feature of this mod: http://newvegas.nexusmods.com/mods/37983 so original credit goes to Imp of the Perverse for providing the means to the creative idea.)