Firstly, I couldn't find the previous post about "Kochal" to see if it was resolved so I'll post what happens.
In the Netherrealm where you fight Kabal, Kira, and Kobra to "Awaken Kochal", if you notice the structure behind Kabal looks as if it could house a portal, which it does. When you beat all 3(Kira, Kobra, and Kabal) a portal opens in the structure. It's hard to see because it's red and blends with the background. Go through it and you come out of the portal where you enter the Netherrealm(the starting point), in front of you is a chest which contains Koins, 1000 koins they were either onyx or platinum, I can't remember which. Kochal is the portal you "awaken".
Now the Unattainable Onyx coin. In Chaosrealm at the beginning, behind the entrance portal is an Onyx koin icon, I've searched the entire perimiter of the starting "island", trying to find a way to get on that side of the fence, but alas, my search was in vain. If you want to see it, stand facing the portal platform, now go to the right, and around the corner to your left, look towards the portal, and you should see the onyx koin spinning behind the portal platform. Also, about the portal design, Stargate much?
Ka-Tra