@ShwiggityShwahApologies for a very late response as I got a part-time job and had to learn the ropes today. So yeah, moving on.
I’m wary of this idea because it sounds like Yidhra is a deity similar to Lucy and Aaurus. This isn’t going to be allowed. It changes the foundations far too much and risks possibly rippling across others. So, I have an idea how to give the same impression without forcing radical changes to the RP structure. It also gives you something to write and explore, while still having that Lovecraftian flavor in the mix.
Right now, the closest thing to otherworldly beings is demons from hell. More specifically a race that hadn’t made its way into the surface. These races don’t fit the myths or legends giving an alien appeal to them.
My idea is that Yidhra is a demon, who is the first of her race to make it to the surface, masquerading as a goddess among her followers. This wouldn't influence or change any of the world's foundation because she's not a higher being, simply trying to pass herself off as one. It's easy to do when no one knows of your race or abilities. I would say she has done this for a few years so she’s not well known but known enough among a set group and other selective individuals.
She infects her chosen avatar with her essence. Now when most demons infect someone, they end up altering the host into another one of their own kind. Some, however, can create a pact. Kitsunes- an example from old essence- were able to make a pact by creating a shard using both their own essence and another’s. This shard bound the two and later prevented permanent death for the Kitsune should they be killed at any time. (There was additional rules such as the Kitsune couldn't harm the other individual or betray them, etc)
Yidhra’s (if the name is straight from a Lovecraftian story, I might suggest using another name and only mimic the theme) pact could bestow a connection with her and possibly a few magical abilities. Thus, differentiating him from the other cultists.
As for Yidhra or her avatar being used as a Bureau attack dog… that’s a little hard to do if they were captured during a raid. Mostly because at this point things aren’t terrible enough for the Bureau to consider using a prisoner (which they would have ways to contain her) or her avatar as assets. And this would place your character on the backburner for a while until things got to that point. Which they won't for a while.
I have two suggests for this part of your concept:
1. Have her captured and imprisoned in the Bureau, making her avatar decide to infiltrate it. Mostly in order to figure out a way to free her. This provides a good, solid and long-term plotline for your character during the rp.
2. Have the avatar willingly join the Bureau then use their missions to keep the demon entertained and gain information about the supernatural world. Another solid and long-term plotline, but with slightly different motives.
No matter what you decide, you need to actually consider a write up for both your character and the demon (using her as an example of her race). Then I suggest you also do a write up for her race, including their culture and their abilities. Mostly for the GMS to look over and understand how they fit into the world of hell for later plots. The race write up is less urgent or important at the moment than the avatar and demon, so it can be written after those two are approved and gradually over time.