How strong are the Nightbrother's physically before and after their infusion with ichor?
Your ask implies what happened to Savage Opress in the aftermath of his Selection was a common practice but I don't think that's the case.
TW: Oppression (Nightsisters), violent death (Feral's)
Ventress and Talzin had a very specific agenda to get revenge against Dooku, and they needed a tool that would be powerful enough in the Force to be considered an acceptable offering by Sith/Dark Acolyte standards, but loyal to them, hence Savage's conditioning (which is why Feral's murder was the final test of Savage's devotion, because if he'd been in his full faculties, he wouldn't have snapped the neck of his beloved brother. Think about that for a second: Feral wasn't choked to death. His spine was broken one-handed. That's not an effortless death.)
I suspect that the Nightsisters aren't so generous unless it serves their ends specifically, because not only was Savage's size, strength, and speed bolstered, but he was also given an ichor-infused weapon (a pike made by some variety of conjuration), and presumably his armour was also enchanted (because it evanesced at the time of his death along with the changes made to his body.)
For the before and after aspect of your ask, things are going to get a bit more speculative from here out because of the details introduced by Ahsoka about the Witch Kingdom of the Dathomiri, and specifically what physiological differences exist between Dathomirian and Iridonian zabrak, and their consistencies.
To date I've assumed that there are similarities between the two species, as both Dathomirian and Iridonian zabrak are warrior cultures familiar to the often brutal conditions of their homeworlds, but Dathomirian zabrak also have the misfortune of suffering the harsh conditions of living beneath their oppressors. I think Nightbrothers were largely self-sustaining and share similar values with ancient warrior-centric civilizations (Sparta comes to mind, as two-second answers go, but this is a tangent) which might've influenced how they trained and fought and lived as a brotherhood.
Apart from the standard: faster, harder, more acrobatic, more resilient, more resistant to pain than humans, how much of that comes from biology and evolution and how much is a product of their aptitude to survive against all odds is up to you to hash out with the new information we're getting.
The tl;dr is that I think Nightbrothers were a hard people whose values allowed them to survive some pretty shitty conditions, but their circumstances didn't allow them to thrive, and there isn't a Nightsister out there who, with an instinct for maintaining the status quo, would give their power to the people they've enslaved.
I do think about what might happen if the Nightbrothers gained control over the ichor, though, specifically with the intention to do exactly what you're proposing, and more. I think about what it might be like if they could wield it. I think about insurgence and uprising, and overthrowing their oppressors. I think about how it might've allowed them to change the game, to take back their power.
In the book of spells recovered from the Nightsister catacombs, several pages were torn at the spine, leaving only the jagged edge of flimsy behind. The presence of these pages, the remnant markings, and the haste at which they were removed suggest the spells were those that dealt with matters of the heart:
Love spells.
It is unclear what became of the pages, whether they were removed by subsequent generations of the witches following Allya and suppressed, or if they were torn by other, hasty fingers seeking knowledge to soothe a broken heart or to compel a lover, or discover infidelity and exact revenge.
The only page that remains partially in-tact contains the remnants of a ritual designed to find one's mate — a dangerous undertaking that involves imbibing the waters of life in the effort to determine the length of time before one should meet one's hearts' intended.
Though the rite is incomplete, the desired effect results in the number of days remaining appearing on the inner arm as a scar, and each day that passes, the number will change.
Scuff marks on Dathomir's gnarled trees indicate areas where young Nightbrothers rubbed their sprouting horns. Bark rubbed off the trunks have no particular patterns, but the placement and regular occurrence of those marks suggest discomfort from rapid horn growth at an early age.
The marks are often found three to five feet off the ground.
The practice of making the ascent to the cave beneath the Witch's Horn fell out of use after 10 BBY. The lair, once the roost of the chirodactyl called Gorgara, retains hydraatis acid markings left by former Nightbrothers who made the journey up the peak.
Startling examples of early offerings have been preserved in the cave: tokens for the chirodactyl made in exchange for the health and protection of the brothers, suggesting a practice in sympathetic magic, or that there was a shared belief that the chirodactyl — one of the oldest of its species on Dathomir — may have possessed magical powers.
Several of the pictographs have been smeared with mushling inks to simulate Nightbrother markings, and many have faded paints in place to suggest variations of goldenrod, brown, butter yellow, and crimson skin. The most daring and elaborate pictographs must have taken hours to etch, the risk to self less than the dangers presented to those Nightbrothers requiring supernatural aid.
Dathomir's "brotherhood", it seems, extends beyond concern for the individual, often petitioning the creature to grant favour to those in need: wellness, good fortune, safe keeping for the smallest and most vulnerable of the Nightbrother warriors subject to the dangers of their world.
Created by centuries of erosion, the Whispering Caverns are so-named for a particular architectural feature that, when a Nightbrother stands at the precise centre of the largest chamber beneath the highest porthole fissure in the ceiling where the starlight is visible through the stone oculus, the walls breathe with the voices of their ancient kin long-passed.
It is tradition that, on a Nightbrother’s thirteenth name day, they visit the caverns alone to hear the secrets of their futures, and to be granted wisdom for their queries: will they live long and die gloriously, will they accomplish great things, and will future generations speak their names in years to come?
Markings left on the walls — names, mostly — recall the brothers who stood in the eye beneath the stars and wondered what would become of them.
Many are faded even though they are not so old, but some still keep their shape after years… or maybe just a few characters that have not suffered the wind and the grit wearing them away like their fates:
In the aftermath of the Battle of Dathomir, in the old Nightbrother village before the survivors relocated, those living there experienced the aftershocks of the curse used by Old Daka:
It was not uncommon for a Nightbrother to wake to odd sounds in the darkest hours, a pale face in their window, or sometimes hovering above their cot in slumber — no purpose nor direction now that the magic persisted, but the one that controlled them did not.
The reanimated Nightsisters that haunted the village after the war were often dispatched unceremoniously by fire or with weights to the bottom of the nearest swamp, and those unfortunate enough to encounter the walking corpses were the fodder for many campfire stories in the years that followed.
Few Nightbrothers hesitated to defend themselves.
Some were the mothers that abandoned them, after all.
It takes nine ripe mushling pods to cover the entire body of a fully-grown Nightbrother with tattoos. They must be harvested beneath a blood moon, the paste made by hand with an exact ratio of seed to water to ensure its not diluted while boiling, the constituents that add colour stirred in carefully by a second set of hands:
Clay for the ancestors.
Ash for the Fanged God.
A drop of blood for brothers lost and fathers unknown.
Recorded on a datalog in the aftermath of the Battle of Dathomir, a visitor observed that the most unsettling thing upon visiting the Nightsister fortress was not that the world had immediately began the process of reclaiming the fallen's bodies — both those that had been resurrected and those freshly dead — but the silence.
The description reads: "Beneath an impermeable, overhanging blanket of mist, Dathomir sleeps — drawing back its people to the cradle of its power to renew itself in the never-ending cycle of creation and rebirth. Already, the dead find their place again at the world's heart: still beating, despite the brutality and the loss of its matriarchs."
Conversely, in the remains of the Nightbrother village, revels could be heard ringing around the mountains at all hours amid burning bonfires and feasting: the shouts and songs of Nightbrothers, old and young, carrying into the dawn after all the mourning dirges were sung.
Even though Nightbrothers are taught to be fearless, there are still places where the overgrowth suggests the spaces are so sacred and so alive with the Force that not even the staunchest brothers venture.
One such place is a ring of stones just past the Dreaming River. Concealed by the verdure's tangle, you might miss the standing monuments that come to the waist height of a fully grown male.
Hunched in the gloom, these stone monuments are said to be brothers put under a Nightsister curse who slumber eternally. There are thirteen in all, and at midwinter, the Twins' setting light aligns just so to illuminate a path through them, pointing into the swamps beyond.
If one is brave enough to follow the path of the suns, however, one might come upon the ruins of an ancient temple where the old gods are still said to walk — close enough to free those stone brothers of their curse, but never close enough.
Initially thought to be votive offerings, the collection of small, hand-carved figures unearthed from the sediment near one of Dathomir's volcanic springs have created quite the stir among experts trying to decipher their origins.
Among them are a palm-sized rancor, a chirodactyl absent one wing, and three partial reconstructions that were presumed Nightbrother warriors -- effigies or commemorative sculptures to the heroes of the earliest clans spoken of in their oral traditions.
Recent studies suggest otherwise, however -- a startling proposition for a warring society so centred around martial disciplines:
The figures' scattered arrangement and proximity to the spring where the youngest brothers were taken to bathe are now thought to be toys.
Among the Nightbrother trials, the process of Selection was the most gruelling. Its purpose was to choose a champion from an adept — to transform them into a warrior worthy of the Nightsister that decided their success.
The ritual of the selection did not inherit the nickname, "The Crucible", but rather, the Nightbrother who emerged victorious was awarded this prize: a transformation through Dathomiri magic by ancient rite.
Archaeological records suggest that the early Nightsisters preferred to submerge their champion in a cauldron filled with ichor to bestow gifts of strength and ferocity upon him, and when the Nightbrother rose from the green mists, through sacrifice or loss or victory, he emerged transformed: empowered and filled with ancient wisdom, a King difficult to control.
Later methods channelled the ichor directly into the soul of the Nightbrother. Perhaps the practice of using the cauldron fell out of fashion, or perhaps the original vessel was broken. One thing is certain: subjects who underwent the crucible in this fashion would not remember the faces of their brothers — only their purpose to serve the Nightsisters...
A Nightbrother could not take revenge against their own under any circumstance. Disputes of pride and honour were dealt with primarily by combat, a tally of infractions kept by clan elders and judged — and only when a challenge was met and a victor declared the offence was forgotten.
In many cases, all two brothers needed was to put some blood on the ground to find commonality once more.
An altar to the Fanged God still in use after six hundred cycles suggested the presence of a haruspex among the last Nightbrother clans.
Thought the practice of divination using the entrails of Dathomir's native fauna were prohibited by the Nightsisters, evidence of animal sacrifice suggests haruspicy was still practiced in secret.
Near the altar, archaeologists uncovered a fragmented tablet containing a prophecy read from a rancor liver: that a zabrak would rise as a warrior to lead the Nightbrothers to freedom and triumph over their oppressors:
He would lose everything to the Dark Side in the process, but the Fanged God would judge him worthy for his sacrifices, and return him to glory.
There has been no record of a zabrak warrior's return to date.
The Nightbrothers don't often manifest after death unlike the Nightsisters, who, with the right talisman can be summoned and spoken to beg farseeing advice or wisdom.
It's speculated that because the Nightbrothers do not share the same bond with Dathomir, they do not maintain the same ties to it in their afterlife as the Nightsisters: it is not their homeworld and the Iridonian cosmovision lends itself to an afterlife far away from suffering.
Simply put, once a Nightbrother passes, they return to the places the soul remembers and loves — a land of plenty where their spirits rejoin their kin.
They can't be summoned because their spirits are no longer bound to Dathomir.
Skilled calligraphers were few in number, but revered for their ability to ink the written word into stone with hydraatis acid.
The Nightsisters prohibited the Nightbrothers from keeping written histories, so much of the legacy of the clans are hidden in places inaccessible by anyone other than the most skilled climbers:
Dathomir's most treacherous peaks and deepest caves bear the proof that the greatest warriors are remembered somewhere, so that the halls that preserve their honour can never be desecrated.
Young Nightbrothers are taught self-sufficiency from an early age. They're taught to hunt and preserve food, but they also learn the land they live on... right down to the plants that might harm or help them.
Bacta is an expensive product and hard to come by, living so far off the hyperlanes, but ask any Nightbrother at the age of five which leaves might draw poison from a wound when made into a poultice, or which roots to decoct to make the strongest potency of bloodroot tea to cure a fever, and he'll tell you where the strongest can be harvested.
The Nightbrothers of Dathomir look after their own, because no one else will do it for them.
The second Nightbrother village was carved directly out of the rock face in the shadow of Dathomir's westernmost mountain range; initially a cave settlement, many of the apartments were equipped with stone furniture, with ventilation shafts providing air for the dwellings.
No one knows how far under the mountain the village descends, because those labyrinthine alleys lead into corridors that unearth more than just living quarters: remains of an apothecary, a blacksmith, and a small, chthonic temple have been discovered, and others are still being uncovered by the archaeological teams excavating the settlement.
There is hope for further surprises, however: Nightbrother sigils invoking protection and a prosperous hunt have been found as deep as eight-five meters, close to the water table.
