As of September 2021, this story is undergoing major revisions and rewrites. I'm also cross-posting to AO3. I'll note the edited chapters as I go along.

The news of his mission had arrived via a letter handed to him by Matsumoto. The fact that she'd dashed from the room the moment that his fingers had brushed the paper should've been a sign of what was to come, but he'd only quirked an eyebrow at her antics before dismissing them as par for the course.

As he broke the seal holding the papers together he could tell from the jagged edges that someone had pried from them a book rather than recopy them— sloppy. The text had a spindly quality to it, not printed from a computer or pressed down into the paper by a deft hand wielding a brush. The text was in English, which added to his confusion, but it was the drawing that caught his eye. It so dark it was a wonder the ink hadn't bled through the page; a hooded spectre in black robes, with claw like hands that almost seemed to reach out at him, and a gaping maw of a mouth that wouldn't have been out of place on a Hollow. He'd seen similar depictions over the years, though nothing quite so ghastly. His finger carefully traced over the title at the top of the page—clearly pulled from some sort of textbook. "Dementors," he read aloud, testing the feeling of the word on his tongue. A portmanteau of torment and demon? It was apt, if so.

His eyes scanned the rest of the page. A monster that could drain the happiness from a person and feed on their very soul— Toshiro stopped for a moment, his eyes going over the sentence again to make sure he wasn't mistaken. But no, these creatures truly feasted upon the souls of the still living and, it was easy enough to extrapolate: the stray souls that had yet to make it so Soul Society or be collected. For centuries they had tried to reconcile the souls that had gone unaccounted for— simply vanished despite no known Hollow activity in the area. It was never a glaring discrepancy; something to wonder after but not devote additional resources to trying to track down. Even to them, death had its mysteries, and this had simply been one of them.

Dementors are primarily located in the wizard prison, Azkaban, where they serve as guards and may be employed in carrying out—

Toshiro stopped. He brought the page closer to his face, re-reading more carefully. A third and fourth attempt at interpreting the words didn't suddenly result in their changing. Wizard prison. His eyes tripped over the word again and again. Wizard. In a move that he'd later deny, Toshiro craned his neck as he glanced around the room, checking for a camera. He even ducked under his desk in search of some sort of microphone that might've been hidden there to catch him in a moment of foolishness. He rifled through the pages, searching for a sign that he was being pranked, before he spotted the seal that he'd broken. It was the official seal of the Captain-Commander. No one except for Yamamoto, not even his aides, had access to the seal. So either the Captain-Commander had decided to develop a sense of humour, or, somehow the more likely of the two, wizards were real. Toshiro took a moment to let the thought seep in.

"Right," he muttered to himself as he straightened his uniform and tidied the papers he'd displaced. "Wizards. Why not."

Most of the Living didn't believe in him, so why couldn't wizards exist? When one final inspection of the room still revealed no recording equipment, he began reading again.

Dementors could apparently be repelled using a spell— a spell— called Expecto Patronum, cast by a witch or wizard using their happiest memories. It seemed simple enough, really, to deal with such a creature more permanently; a simple konso to release all of the absorbed souls would probably be enough. The complicating matter was that they were under the employ of some Ministry in their prison. It seemed cruel, in his opinion, to subject even hardened criminals to having their souls and happiness fed upon. The truly worrying development was that there were currently plans to deploy the Dementors into the general public in search of an escaped convict— an edict issued by one Minister Fudge, of all things. Even with the briefest skim of the information he'd been provided, Toshiro could tell that corralling the Dementors and ensuring that they wouldn't attack civilians would be near impossible.

They seemed far too similar to low level Hollows, driven by hunger and instinct, to manage any sort of higher function that would help them discern between the guilty and the innocent. The highest concentration of Dementors were even set to be stationed at a school called Hogwarts, where it was believed the convict, Sirius Black, might be headed.

Toshiro continued to flip through the information packet, thinking it to be straight forward enough, if beneath him. A few seated officers, or even several unseated, could take care of the creatures given time, if eradication was the desired outcome.

He rifled through the pages until he found what he was looking for: a signed letter from Yamamoto himself.

Hitsugaya-kun,

After carefully reviewing the intelligence that has been collected regarding the creatures known as Dementors, it has been decided that while they remain under control, no direct action shall be taken against them.

However, the situation warrants monitoring to ensure the safety of the Living until such a time that the creatures have returned to their post. Therefore, you are so ordered…