An Obscurial is a young wizard or witch who developed a dark parasitical magical force, known as an Obscurus, as a result of their magic being suppressed through psychological or physical abuse.
According to Dumbledore, an Obscurial can possibly be healed by replacing their feelings of alienation which ultimately created their Obscurus, with a sense of belonging.
When a magical child is forced to repress their talent through physical or psychological abuse, they develop a parasitical magical force inside of them called Obscurus, resulting from their strong emotions of distress.
An Obscurial may lose control when they reach their emotional and mental breaking point, releasing their Obscurus as an invisible (or nearly invisible) destructive wind. In extreme cases, they may physically transform into an Obscurus.
When an Obscurial releases the Obscurus within, their eyes become pure white and their physical form vibrates and becomes distorted. While the Obscurus is invisible, the Obscurial can physically interact with the world without making physical contact, though the ability is uncontrolled and triggered by the host’s distress, anger, or other intense emotions. This notably damages structures and objects near the host.
Usually, children possessed by an Obscurus almost always die before their 10th birthdays, but as exemplified by Credence Barebone, a wizard or witch with tremendous latent powers can survive longer. When the child dies, the Obscurus vanishes along with them, though Newt Scamander successfully kept one alive when its host died by encasing it in a magical bubble. It seems that Obscurials can’t be killed in Obscurus form; the Obscurus may be shattered temporarily, but the Obscurial themself will still be alive if Credence is anything to go by. However, he is only one example and is stated to be unusually powerful.
In ancient times, Obscurials were more common, when wizards were hunted by Muggles. However, when the wizarding world and the Muggle world separated, the trend dropped as children were no longer forced to hide their true nature among their own.
The obscurial Newt met first was an 8 year old Sudanese girl who died, which was another reason he recognized the mark of the obscurial on Senator Shaw.