This reminds me of an old idea I had for a dnd character once and I can probably adapt it to this
A warforged built by a cultist to server his god. He was found just before the machine was officially set loose upon the world, and the robot was put in a coffin or some other hidden compartment, and found way after. This machine was "coded" for lack of a better term to have forbidden magic up its sleeve, but doesnt really know how or why.
All he knows is that he has a weird innate ability to use these forms of magic, but for some reason he can't use any others. Likely because hes a literal robot and shouldnt have magic abilities anyway.
How viable is something like that?
That would not be viable at all for a freshman student at the Academy, but if you want to play a student who, like one of my characters, has parents in the Cult, then you could say the warforged was first built by a follower of Tiamat centuries ago, was placed in a coffin like you said, probably by followers of Bahamut, and then was somehow forgotten about. Centuries later, members of your character's family discovered him and now he's like a guard who is usually posted outside the Cult's secret headquarters. But when the five dark instructors go to the Secret Arena, they could bring him along keep an eye on the entrance and maybe help train the students.