I was stuck in that spot for a while.
I kept falling into the poison fog thinking I probably was supposed to 'sacrifice' life wading in... I was wrong, and often!
Ok, if you already reached the 2 rune spots, then you must have spotted that island you doublejumped to reach that big titan. Well, in that island you can also doublejump to the right. While it seems like an empty little cave with some corpses in it, if you keep on moving into the cave at the far end of the camera, the camera will shift viewpoints. Turns out, there's an alcove in there where you can fight some skeleton coffin things, I believe. Kill them and you'll get a rune piece.
Then return to the titan, use the rune piece to move a piece of the titan, and continue on.