Dunno if this is well-known, but you don’t need to go through the Harpoon temple to enter Shadow Gate. You just need to obtain the Rapier. Jump slash with the secondary and dash to get to the platform (you gain just enough height when you slash off an object). The harpoon is found where it usually is after the cage cutscene. Don’t ask how I found this out. Too embarassing.

Which object do you slash ?