At least some content in this article is derived from information featured in Fantastic Beasts: The Crimes of Grindelwald. As such, spoilers will be present within the article.
An Obscurus is the manifestation of the repressed energy of a young wizard or witch (known as an Obscurial). Described as a "dark" and "parasitic" force, an Obscurus is created when the child in question consciously attempts to repress their magical abilities or were forced to do so through physical or psychological abuse. This energy can manifest itself as a separate entity that can erupt in violent, destructive fury.
When an Obscurial loses control, the Obscurus force is released, often focused on the cause of the Obscurial's torment, but also causing destruction in its wake.
The Obscurus can be largely invisible or appear as a flying, incorporeal, and amorphous entity with the appearance of a black, fluid-like cloud, or a violent torrent of darkness, sometimes with a reddish core. Some describe it as a "dark wind" or "dark mass" with "shining white eyes". Its appearance can be oleaginous, or "rich in, covered with, or producing oil, or oily or greasy". There have also been instances when a face briefly emerges from the shadowy entity, or a white glowing image of the host appears inside of it.
The size and might of an Obscurus depends on the innate power of its host; the more powerful an Obscurial, the more powerful their Obscurus. When an Obscurial reaches their emotional and mental breaking point, they can lose control completely and physically transform into an Obscurus. Its size and mass at specific times may also be connected to the level of negative emotions that triggered it at that instance, as well as the current mental state of its host.
In this state, they are enormously destructive to their surroundings, to the point of causing objects to levitate or break without physically being in contact with them. This notably causes damage to structures and objects near the host. However, they can be calmed down by someone they trust, which causes them to revert to their corporeal state, though being emotionally conflicted can cause the Obscurial to transform back and forth.
An Obscurus often targets people that are the source of their host's distress, as Credence Barebone's Obscurus went after Henry Shaw after he was insulted by him, though it took longer to attack Mary Lou Barebone since Credence was so emotionally close to her. When his adoptive sister ran in fear of him and Gellert Grindelwald, in the guise of Percival Graves, stated that he had lied to Credence about taking him in as a wizard, Credence lost control completely of his Obscurus, causing wide-spread damage in New York in December 1926.
An effective defence against Obscurus and death from his hands is a very strong Shield Charm. Gunnar Grimmson did that in Paris in September 1927 when he killed Irma Dugard, which enraged Barebone to the extent that he wanted to kill him.
When an Obscurus kills someone, it leaves specific marks on the victim's body, largely focused on their face.
It seems that Obscurials cannot be killed in Obscurus form; the Obscurus may be shattered but the Obscurial themself will survive as Credence Barebone did in December 1926 after being assaulted heavily by spells cast by MACUSA Aurors, in their attempt to destroy him. However, Credence was noted to be an unusually powerful Obscurus, which may well possibly have contributed to his remarkable survival.
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