Seems like 9th level it is, then.
ExisD, I would say.. a refluffing of a spell-casting class like warlock or sorcerer or wizard could work. You'd be an alchemist that mechanically functions as the base class for the most part. I would say that spells couldn't have a range beyond 30 feet (as far as you could chuck a bottle, pretty much) and obviously things that couldn't really be the product of a concoction wouldn't be possible. IE; Call Lightning would not be appropriate, but Darkness could be refluffed as being an alchemical smoke-bomb. Cure Wounds or Contagion would be something you pour on somebody as a touch.
Some things about the core races, classes.
Humans and Tieflings are the only common race. Everything else is considered uncommon and are generally not as fertile or longevitous as they once were.
Dwarves have largely gone into hiding underground after an alliance of all the goodly races was crushed by the possessed Duke of Lefure, who paved the way for the demonic scourge.
Elves remain above-ground, in small communities usually cloistered high up in the mountains. People assume they are still up there, but most people don't know for certain.
Wood Elves are teetering on the brink of extinction- they did not participate in any of the wars but have been stubbornly trying to keep to the blighted forests. They have become almost the subject of superstition, and like all things associated with the blight, are considered cursed and tainted.
Halflings are a little more common than their non-human brethren, mostly because they've clung to human lands. Still, there's not too many of them left. Most have settled down, the roads becoming much too dangerous. Others have made veritable rolling fortresses out of their caravans.
Or gnomes. They are even more elusive and mythic than before.
Half-elves are relatively more common than their elven parents. But they're still few and far between.
No half-orcs. No orcs, for that matter.
Tieflings. They might compose... 20% or so of the world population. But considering that there were next to nil 100 years ago, that is a huge growth spurt. They are the product of a deliberate attempt to muddle the gene pool of the surviving population so that the Lords of Men can control them better. While the original generation was most likely entirely the product of raping female humans, subsequent generations more and more often have either tiefling mothers and human fathers.... or both parents as tieflings. The demons breed them like rabbits, and will always prefer a human sperm donor to a human egg donor on the basis that they can more easily control a tiefling mother than a human one harboring a half-breed in her womb. This means that most tieflings today are born under the yoke of the Lords of Men. Still, many have escaped or are the product of several generations removed from the original rape.
The tieflings who live amongst humans are often mistrusted, even if they were born within that community. Among more irrational communities, they are usually the first scapegoat whenever something goes wrong. Tiefling strangers are almost always considered some sort of spy for the demons... which is often a very good assumption, as they are used for that purpose.
Barbarians cover a wide range of peoples. Most of the times, it does not describe a certain tribe or way of life, but how one merely fights in the face of adversity. A barbarian could be a town's man-at-arms who ferociously fights the demonic horde.
Bards are in fact very common. Their ability to make light of a bad situation and bring people together and liven their spirits is in fact very valued. Mechanically, they would not have any spell-casting. Or their spell-casting would be a form of alchemy that has limited effects.
Clerics are just about all gone. Magic still exists in some slight sliver, but it is unreliable. Divine magic even moreso. Even if someone was born with the gift for divine magic, the odds that they still believe the Gods exist is slim to none. Most people will say that the Lords of Men are gods and old pantheon has been completely forgotten or considered a false religion.
What few Druids that remain have become enigmatic and rare things. Most are half-mad, driven to insanity by the blight. Most are wood elves, reluctant to leave their thousand-year homes. There are few 'new' druids being initiated. Most nations on the edge of the Great Forest are completely deserted- that's how terrified of it people are.
Fighters are... well, fighters.
Monks were pretty common in the East, but as the Great Forest has split the continent of Loth Carnon in half and the seas are pretty perilous, many people occasionally wonder if their far eastern brothers are doing any better... When the kingdoms of Kiampur and Sangkur fell to ruin at the spread of the Great Forest into the south, there was a mass exodus. Many died on the seas, while others landed in Lefure. Unbeknownst to them, already under the rule of the demon-possessed Duke who would later proclaim himself first of the Lords of Men.
Paladins are largely gone like their cleric brethren. The old knighthoods have long histories that were mostly expunged during the wars against the Duke of Lefure. Vaelor Roxaneis was already considered one of the last paladins alive when he died in combat against the Duke. He has since become undead and may be the last remaining paladin alive. He has made little effort to keep his tradition alive, as his own power has significantly waned.
Rangers are both valued and feared. The blight poses and immense risk to ordinary people. One false step can lead to infection and a gruesome death. Even the non-predatory animals there pose a threat. Most of the food in the forests is tainted... but to those who can tread the blight, to those who can forage that fraction of untainted food from those depths- their skills are invaluable. Still, most of them are considered... 'creepy' I mean, what sane person would willingly go into that abomination-filled wastland? Oh wait, that's all the PCs.
Rogues and Fighters are probably among the most common classes. Both are different paths towards the same end- surviving. Many tieflings become rogues, trained as such to infiltrate human settlements or because their neighbors' prejudicial nature has made burglary easier than the straight and narrow path.
Sorcerers and Wizards are just about all gone. People believe they existed at one point, sure... but now a days they exist only in cultural memory.
Warlocks are often tieflings or devil worshippers, granted their powers by the Lords of Men themselves. Archfey warlocks are much like druids, considered mad and repulsive. Great Old One warlocks are rare, but they exist. They draw their power from... somewhere. Not even the warlocks themselves know where this power comes from.
So, as you can tell.. Magic simply doesn't exist except in very rare circumstances. Most of those circumstances are also derived from the fiends themselves. How to implement this mechanically, I am not sure. For the moment don't get too attached to the idea of playing a spell caster or having magic items.