Found it, and it's kinda mean lmao. The one you did is the West sigil, and you're missing the East one. Pretty sure you already found it as well by now, but just in case someone comes here looking for it like I did:
You know the first arena to the left once you enter the temple, right? The purple smoke that locks you in that place and spawns monsters before you can go up the ramp and the platforms? Well do that and go up them, but before you jump on the platform where the second arena is, you can see that there's a small ledge to the right side of the wall. That's the path.
Once you reach the pillar, go to the left platform first, it has a lever that unlocks the door, then go back and drop to the right platform. Now it's just a straight path towards the sigil, but here's like, three arenas with pretty strong monsters until you get to it, but once you do you can return to the main area freely.
That's the last quest as of 9.2 btw.