Were you struggling with finding the last Obsidian Skull in the game? This guide will show you where you can find a portal for this collectible. Basically you can find it after reaching the checkpoint, particularly in the place where you need to press the red button to activate the gate. The portal appears on the platform that you can find on the left (when you approach the moving platforms). It is required to use the grappling gun to reach this portal.

Since the game features the photo mode, I'll simply display it in the picture below.