At least some content in this article is derived from information featured in Harry Potter: Wizards Unite. As such, spoilers will be present within the article.
For gameplay-specific information, please see the relevant article on the Harry Potter: Wizards Unite Wiki!
A mysterious organisation that was involved in the Calamity, the Unforgivable communicated through notes delivered through Foundables and false memories. These communications revealed that two members used the codenames Lethifold and Barghest. "Barghest" was possibly the codename of Grim Fawley, a suspected member and the person originally accused of casting the Foundables Spell.
In the winter of 2020, the Unforgivable began a campaign to "bring them back", which was believed by Constance Pickering to be a plot to resurrect Voldemort and his Death Eaters. Through the Unforgivable's manipulation of the Calamity and their subsequent access to Harry Potter's memories, famous Death Eaters began manifesting as a new form of Foundable known as Adversaries.
In the winter period of 2020-2021, Muggle journalist Brice Badeaux became witness to multiple Foundable traces and reported them online in a serious breach of the International Statute of Wizarding Secrecy. This was in spite of Statute of Secrecy Task Force members taking all precautions against discovery. When caught with a Triwizard golden egg Foundable, he attempted to run away with his evidence, but was caught and obliviated. Brice seemed to have foreknowledge regarding the Wizarding World and knew that he would be stopped and obliviated, raising concerns that he had been aided by the Unforgivable.
In the spring of 2021, the Unforgivable threatened Harry Potter and Hermione Granger after the two began wrenching control of Calamity surges away from the cabal. The two feared that their children would be targeted, but Harry insisted on continuing to fight against the Unforgivable. Harry redirected a Calamity surge and controlled it using memories of the Marauders. When Minerva McGonagall was uncharacteristically late in reporting to the SOS Task Force, Constance Pickering, Harry Potter, and Hermione Granger feared she had been attacked. By April, it became apparent that while Minerva was unharmed, Ginny Weasley had been attacked and Ronald Weasley had been abducted.
Following their attack on Harry's wife and Hermione's husband, the Unforgivable unleashed a surge of Dragon Adversaries. While this was disastrous, it allowed Harry to tap into somebody's memory of being forced to abduct Ron. Shortly after, the Unforgivable unleashed a surge of Foundables related to Weasleys' Wizard Wheezes where Ron worked. While the Unforgivable succeeded in intimidating Harry and Hermione, Harry even enlisted the help of MACUSA in the hopes of finding his best friend, this attack only made Hermione even more determined to bring the cabal to justice. The Unforgivable began a campaign of harassment and manipulation to secure their position preceding negotiations for Ron's returns, including implanting false memories in Molly Weasley of Ron crying and begging her to tell Harry and Hermione to listen to the Unforgivable so that he could return home. Molly instead instructed Hermione to "give them Hell." 
By the end of April, the Unforgivable had delivered their list of demands via notes dispatched within Foundables. In exchange for Ron's safe return, they not only wanted full command of the Calamity but for London Five victim Kit Gerrard and the defector informant Flavius Flint to be delivered to them. Hermione decided that she and Constance should use Polyjuice Potion to take the places of Gerrard and Flint as a means to infiltrate the Unforgivable, with Harry delivering them in exchange for Ron. It seemed the Unforgivable were aware of this deception, as well as where the Ministry's defences were weakest, however.
Whilst duelling against members of the cabal, Constance was hit with a Blasting Curse but luckily came away with only mild burns. While Ron was recovering, albeit unconscious, the real Flavius and Kit were taken. When Constance returned from having her injuries healed at St. Mungo's, she reported that Ron had woken up and his memories had been completely Obliviated (to the point where he could not even recognise his wife. Hermione wondered if everything, including rescuing Ron, had been part of the cabal's plans all along. She and Harry considered the possibility of a mole in the Ministry or a Legilimens among the Unforgivable's ranks. 
In May of 2021, Calamity surges surrounding baby magical creatures created the impression that the Unforgivable were involved in illegal poaching and trading of rare beasts. Fragments of a note apparently from Grim Fawley contained co-ordinates to a location in Cape Town, to where Harry Potter led a team of Aurors in order to stop a potential auction, and to find members of the cabal. Unfortunately, no auction took place, adding weight to the theory of a mole in the Ministry. The return of Mountain Troll and Acromantula fragments caused Ron Weasley to remember events from his past, though he still could not remember the names of people close to him. Following this, Hermione resorted to drugging various Ministry members with Truth Serum then erasing their memories of being interrogated by her. This method outed Gareth Greengrass as not only the mole but a high ranking member of the Unforgivable.
Hermione hoped that the detention of Gareth Greengrass would put a stop to the appearance of Adversaries, however instead the SOS Taskforce was inundated with an unprecedented surge of all known Feared Adversaries over the course of a week(14 - 21 July 2021). Hermione speculated that perhaps Greengrass was capable of manipulating the Calamity without a wand, though Constance thought it might be that Greengrass was not the one responsible. Hermione also claimed that "anyone who could bring Darkness like this into the world must be capable of powerful magic." 
Following his detention, Gareth Greengrass repeatedly proclaimed innocence and his colleagues in the Department of Mysteries called for his release, even going so far as to accuse Harry and Hermione of using the Imperius Curse to obtain a false confession. As testimony procured under Veritaserum was inadmissible and there was no paper trail or actual evidence of Greengrass's involvement with the Unforgivable, Hermione turned her attention to helping Ron recover his memories of the 1992-1993 school year. During this time, an Auror discovered that Gareth's strange habit of delivering Interdepartmental Memos via owl was part of a ruse. Gareth's owl was a Transfigured potions cabinet containing Polyjuice Potion. When tested, those who imbibed the potions turned into Hermione and the editor-in-chief of the Daily Prophet, linking Gareth Greengrass to the abductions of Ginny Potter and Ron Weasley. A complication arose, however, when it became apparent one of the Polyjuice Potions had been tampered with during the month that Gareth was detained. Hermione concluded that this was part of an escape attempt and alerted Harry.
Magical abilities and skills
- Dark Arts: The Unforgivable were seemingly able to use dark magic to tap into the Calamity and control it for insidious purposes. Members of the Unforgivable also used the Blasting Curse to injure Constance Pickering whilst duelling her.
- Charms: This group was known to use Memory Charms to wipe clean the minds of those with incriminating information about their activities.
- Duelling: Members of the Calamity were shown to be proficient with martial magic and magical combat.
- Harry Potter: Wizards Unite (First mentioned)
Notes and references
- Harry Potter: Wizards Unite
- Harry Potter: Wizards Unite (Brilliant Event: Triwizard Secrets)
- Harry Potter: Wizards Unite - Brilliant Event: The New Marauders
- Harry Potter: Wizards Unite - Registry: Adversaries Events
- Harry Potter: Wizards Unite - Brilliant Event: A Weasley Predicament
- Brilliant Event: A Weasley Predicament
- Brilliant Event: Creature Discomforts
- Brilliant Event: Unforgivable Truth
- Harry Potter: Wizards Unite (All Feared Adversaries Event)
- Brilliant Event: Secrets Revealed
- Harry Potter: Wizards Unite (Brilliant Event: Darkness Rising)
- Harry Potter: Wizards Unite (Brilliant Event: Dumbledore's Army)