I'd feel ashamed linking it to the other, much more complete Infernal projects; you can jump to those over at InfernalExalted. I'll put it there perhaps when there is more available.
My conception on Infernal Exalts is something shaped by the Technocrats of Mage- rather than the Nephandi, which are the obvious choice. Somewhere in 'my' timeline, the Infernals would break free of the Yozis; more specifically, the Alchemicals would be cut loose from Autocthon as he went insane at the ending of the Age. The Infernals, already teetering on the edge of freedom, would be 'modified' by the surviving Alchemicals as a source of power; the Yozis would of course protest, but they'd have rather their own problems at the time. Being the (twisted) Solar Exalts they are, the Infernals would slowly dominate the Alchemicals... leading to the World of Darkness future history in which an order of golem shapers and crafters of magical prosthetics (the silver hand of Celtic myth, for instance) end up as HIT Marks.
But Mages are Sidereals?!!?!!!111- eh, who cares. The severing of Shards means there's almost certainly some cross-pollination between Exalt types leading into the supernaturals from WoD. If it helps any, I borrowed parts of this from Demon. To be wholly true to Demon- and in some ways, perhaps a more interesting scenario- Infernal Exalts would have to be twisted gods and elementals.
Chewing On Madness
That stated, the Infernal Exalts of the Age of Sorrows are also inspired somewhat by Victorian-era occultism. To my mind, they are elegant, gracious and utterly ruthless. Most are clad at all times in dark or reflective glasses, long (though not necessarily heavy) dark clothing and gloves, their genteel attire masking bodies savaged by the twisting of the Yozis. In order to make their operatives easier to control, the Yozis, masters of soulcraft, surgically remove the animal soul of their future Exalts. This involves a Malfean geomantic realignment for the Twisted Ones' bodies- the skin under the shell the Exalt wears is horribly scarred and mutilated. In some cases, demonic prosthetics or other, stranger organs have been added to the Infernals' bodies; some Infernals might perhaps not need to eat, draw breath, or even particularly care if their head was severed from their body. Perhaps one who watches closely the shell of an Infernal may see... twitching, beneath the cloth, or a slow rythmic pulsing as a demonic heart beats, causing the entire torso to convulse.
Powerful or specialized Infernals would not need the cloth and glass shell of their bretheren, instead crafting a fleshly shell to disguise them almost perfectly into the mortal world. In all cases, however, the anima banner would slowly reveal their true nature; probably creating an effect like the Eyes of Madness merit/flaw from the PG.
Breaking Free, Going Under
My basic conception of Infernal Limit is that it is something similar to Resonance. Alchemicals gave me a new way to look at it; the inverse of Clarity/Dissonance. Infernal Limit is, instead of a measure of their personal Curse (the Yozis would not be so clumsy as to leave that in place over their Chosen), the measure of how much free will they have. As their Limit increases, the control their masters have decreases- and the pain returns. Rather than the absolute Clarity of demonic conviction (to use a term loosely; I suspect Infernals would have rather different Virtues until their Limit Break), the mutilated once-human creature would begin to feel the physical, then mental and spiritual agony of their condition. The Break for an Infernal would result in the destruction of their shell by their body as the various demonic limbs, wings, tails, organs and whatnot rip, melt, burn or otherwise destroy the covering to reveal the true horror of the Infernal's form- just as he or she also regained a memory of their human days and the remaining touch of mortal virtue. For most Infernals, the revulsion and horror others experience in their true form's presence is enough to re-commit them to the Yozi's cause.
However, redemption, even for ones such as these, is possible. There are permanent marks left by the hellish surgeries that created them, but the Infernal could rejoin the weave of fate with appropriate assistance from a Sidereal and a Solar. One who wished to again appear as human might also seek the services of a Lunar. Living this way would mean embracing near-constant pain and the knowledge that one is still different from everyone else around him or her, but an Infernal who had been brought back to the Weave and dies free in Creation returns his Solar Essence to its proper place. This should be very nearly impossible, and a stipulation against suicide makes it all the better- for can suicide truly be called 'dying free'?
Again, this is far from complete. Rantings of another madman.