Is there anything existing I can use for this idea?

[tchntm43]

I am not sure if there is an existing monster that has been created for an idea I have.

For background, in a previous adventure, the party had freed some hostages, and they left the hostages alone to pursue an enemy. The hostages tried to find their way out of a swamp and past a long stretch of open grassy hills. Two hostages died, and the party found a body on the way back to town as evidence of that. However, the body was next to an ankheg that had also been mercilessly ripped to pieces, and there was no explanation of what happened, and the hostages refused to talk about it, creating an unsolved mystery that I intended to come back to later.

That later is now. I want to create some kind of evil plague, except the disease is not a normal kind. It finds individuals who are under duress and possesses them, imbuing them with a sort of madness that includes super-human strength and constitution, but also a bloodlust to kill anything in sight other than other infected individuals. When the killing is done, the plague leaves the infected behind who only remember trace images that are traumatic, and flows away like a fog. In a way, it's like being afflicted with being a werewolf, except that the people aren't permanently afflicted and it's a separate entity that passes from one group of people to another.

I'm not sure if something like this exists in D&D lore already. I can create it if I have to, but someone else might have thought of good ideas for it that I would miss.

[The Glyphstone]

That doesn't sound like or act like a plague or disease in any fashion, even a magical disease. Rather, it sounds a bit like how the Loumara behave - a grouping of incorporeal demons that possess people or objects adhering to a specific set of criteria and compelling them to do certain things. There isn't a specific existing Loumara that acts that way, but you could easily homebrew one based on the printed examples in Fiendish Codex I.

[NRSASD]

Take a look at Shadow Demons. Failing that, Mordekainen's Tome of Foes has a ton of demons and devils that might fit the bill. Personally though, that sounds more like the work of a fey to me.

[Man_Over_Game]

It's slow going, but I asked this question on RPG Stack Exchange to see if anyone had any suggestions: https://rpg.stackexchange.com/questions/151574/is-there-a-monster-plague-that-makes-people-go-berserk/151581#151581

Nothing definitive yet, though. Apparently 3.0 had a Festering Anger disease that made you sick, enhanced your strength, and made you attack things that made you angry, or even reminded you of something that makes you angry.

[tchntm43]

Thanks for the responses. Demonic possession is something I'm thinking about as well. I'm playing in a homebrew setting, so of course I can change any aspects of things to fit what I need. I'll look into some of these for further inspiration.

Demonic origins might actually work out in connection with other events in the setting. The nation to the north is in a sort of cold war with the nation the party is in, and it is ruled by an evil arch-wizard who has formed a pact with a demon prince to create an army of warlocks (in this setting, warlocks exist exclusively in this role). So this could be a new subversive form of attack by that dude. Hm indeed.