I think I am going crazy.

I added the "Book of Ancient Secrets" warlock invocation to my Genie lock. A spell showed up on the dropdown list for a ritual: Mirror Blades. Its description is as follows:

"The mirror blade spell allows the caster to take a dagger and magically make it disappear with a hand flourish. The dagger can be summoned at a later time as a bonus action and be magically directed at a target within 60 ft. along with a number of mirror daggers equal to the characters total number of levels. These daggers speed towards their any number of targets that the caster chooses and they hit automatically in the same manner that magic missiles do. The dagger and the mirror blades can be expended all at once to any number of targets as a bonus action, or they may be reserved for another bonus action during another turn until a number of mirror blades equal to the character's level have been expended." 

Sounds great! I add it. But then I get curious, because I hadn't heard of that ritual spell before. So I look at the source. It just says "Version A." So I think, oh, this must be homebrew. I go back and check and I have the Homebrew options box turned off for my character.

Weird.

Out of curiosity I uncheck every other optional box: Critical Role, Magic the Gathering, Eberron, Dragonlance, Rick and Morty, Noncore, etc. I refresh and go back. Yup, Mirror Blades still there as a selectable option. Huh.

I use the search option on Dndbeyond for "Mirror Blades" and get....nothing. Wait what? It's a selectable option on Dndbeyond but Dndbeyond says it doesn't exist in their archive? I think maybe it's a weird bug and homebrew got through. I check my own homebrew archive. No such spell as "mirror blades" in my own collection. I select to browse through Dndbeyond homebrew and search "Mirror Blades" and again, nothing. I google "Mirror Blades" and "Mirror Blades 5e" and "Mirror Blades Ritual" and "Mirror Blades First Level Spell" and a variety of other forms of that same phrase and get...nothing. Not even a reference to someone's homebrew. The closest I could find was this forum entry from 2011 about a different version of the Marilith monster; it shares the same name and general concept but isn't the text of the spell from above: 

"Mirror Blades: Once per day, by performing a one hour ritual, the Marilith may create up to five perfect copies of any nonspecial melee weapons she's wielding. Those copies instantly crumble to worthless dust if they leave the marilith's possession."

I look up the official 5e monster on Dndbeyond. No reference to mirror blades. I manually look through every source book. No spell named "Mirror Blades."

I do find a "Blade of Broken Mirrors" item from Critical Role, but it has no reference to the spell name or text above. 

I am losing my mind. I can't figure out where this spell came from or if I can use it. At the moment I took it off because I'm thinking this must be a bug. But I can't find the spell anywhere by search options or even reference. Please save my brain. Taking the Book of Ancient Secrets has proven too much for it.

Are you in a campaign?

Homebrew is automatically shared with everyone in the campaign. And yes, homebrew spells can appear in some options even if homebrew is turned off. One of the many and ever growing list of things D&D Beyond promised to fix and never did and probably never will.

I am! So perhaps it's a thing the DM or a player added to their homebrew collection from somewhere (or made up, although that seems unlikely for this DM) but is not generally searchable?

Quote from ChickenDenchers >>

I am! So perhaps it's a thing the DM or a player added to their homebrew collection from somewhere (or made up, although that seems unlikely for this DM) but is not generally searchable?

 
Aye. You don't have to publish the homebrew for it to be shared in a campaign.