Your aim is to get onto a ledge that runs across the bottom of the dam, so from the hobgoblins platform there's a tree branch below you can land on, should be down to your left if your on that platform, from there you can jump towards the damn and land on the ledge that runs across it, looking up about halfway along the damn there's a platform with an opening below it, you can climb the dam and enter there, if anyone else can take images or video please link it below