Nocturian

Technically Ahriman didn't hold off Magnus it was only the intervention of tzeench that kept him from dying but he's grown far more powerful since then

True, Tzeentch did intervene, I just like to think Ahriman did at least something to defend himself.

I mean, he was just done performing the entire rubric of Ahriman, imagine how furious Magnus was at that point xD.

It's surprising Magnus didn't kill him anyway with just how mad he was even tzeench would've had a tough time stopping him then