The first youma were accidents, victims of misfortune and accident. Even during the brightest time of the Imbrian Empire there were places on the Moon that were dark and dangerous. A small group of travelers, it would only take one small incident, a misfired spell, a miscalculated conjuring, and then a manservant was a slavering monster, or a scullery maid was chewing on someone's skull with a mouthful of crocodile teeth. It was eventually discovered that these rare incidents could be replicated, deliberately, with the proper, or improper magic incantation. In the dark times, fallen magi would use this loathsome magic to turn innocent victims into horrific monsters and use them as weapons. The Empress in her great wisdom banned such magics and made possession of these spells a crime.
Youma are highly variable in appearance, depending on what sort of energy shard is fused with the victim. Most Youma retain two forms, one their original human form, and the other a horrific distorted monstrous form that can have almost any shape, or characteristic. There are common themes to the appearance of a youma, however.
Animalistic Youma have a hybrid form that combines the physical attributes of an animal with a humanoid general shape. These youma typically favor mammalian forms, with lots of teeth, fur, and claws, but reptilian and piscine youma are not unknown. Atlantis, on Earth was more prone to encountering the water breathing Youma, and places like the colonies on Mars more often saw fur and scales.
Demonic Youma have stereotypical demon features, horns, claws, wings, glowing eyes, etc and range from mindlessly beastial to being very devious and cunning. It is also not uncommon for these class of youma to have elemental and magic abilites such as breathing flame, poison, or doling out dark sorcery.
Alien Youma produce writhing masses of tentacles, sucking tooth filled orifices, and chant strange insane melodies. These are thankfully rare, but when they do appear, they can be among the most powerful and the most dangerous.
Youma are created through black sorcery which conjures a celestial shard and implants a weakened, diseased, or otherwise compromised host with it's alien energies. The resultant chimera is neither human nor shard any longer, and instead rises as an insane monster that is bound to the will of the Magus that created it. The power of the youma created is inversely proportionate to the individual used as it's host. The largest and most intimidating youma have been created not from strong and competent victims but rather the very young, the frail, or the elderly. This has created a very unfortunate condition where dark magi will abduct children and turn them into youma after an apparent 'kidnapping attempt'. The shard will pupate inside the child and when the monster erupts those who would dispatch the beast are stopped in their tracks because there are very few who are so stoic and resolute that would cut down their own child to slay the monster inside.
Killing a youma is typically a straightforward process. Weapons imbued with magic, made of moonsteel, and magic are typically very proficient at destroying a manifested youma. The youma is also vulnerable to conventional weapons and means of dispatch when the human host is dominant, and this is typically the easiest way to kill them. There are specialists who work in tracking and identifying the human hosts of youma and assassinating them in such a way that the monster doesn't have a chance to assume it's semi-divine form.
Youma do sometimes, but not always, possess means of evading death and destruction. Many demonstrate the ability to regenerate, growing back severed limbs, healing injuries, and such. These youma require acidic spells, flame, and specific types of poison to to permanently kill. Others have demonstrated the ability to shed their outer husk and escape, split in half and create copies of themselves, or spread their condition through fluid or energy transfer.
Powerful magics can be used to 'cure' a youma. This sort of magic is powerful and dangerous as it requires the youma to be under the full sway of it's monstrous form and then drawing it out and fighting it until it is weakened and it's energy is close to exhausted. Then, powerful divine spells, the sort known by the Empress and her Royal Templars.
The Little Girl Bomb - A villainous nethermancer has adopted a pattern. Before his minions attack an outpost, there are a number of children who appear and are taken in. Most of these children are frightened and tired from being exposed to the elements, but one of them is a lurking youma. In the dead of the night, the creature emerges as a horror of clawed tentacles, spewing poison and acid and tearing through soldiers like they were wheat. How many times does this happen before children start being persecuted for fear of being a monster?
Youma Assassin - a cunning youma has made a living for itself by working as a professional assassin. Few suspect this gray headed old woman of being a threat, not knowing that when the opportunity presents itself she can stand up from her rolling chair and turn into a six armed fur covered engine of death.
Additional Ideas (1)
The Cosmic Era
The Cosmic Era has incidents during which human beings are compromised, mutated, or otherwise severely altered by the strange energies and anomalies that boil up during dimensional fatigue events. The technical term given to these individuals is 'Wight'. They retain their human appearance but develop inhuman abilities, with the most common being supernatural durability. Most Wights are able to ignore anything less than military grade hardware. Enhanced strength, parapsychic abilities, and strange biological adaptations (tentacles, new orifices with alien teeth, claws) are also present.
The Wights that are spawned in the Cosmic Era do not follow the trope of shapeshifting into monsters, and maintaining two forms. If a wight manifests a new more horrific form, that becomes it's new form. The standard Youma is based more in the anime tradition of transforming monsters, but also functions in a much more magic setting.