Unchained spider
Updrafts.
It’s not the one needed for the collection. Do the event at the Lyssa shrine to power it and it’ll spawn at the end.
For the others, for those who come across this thread:
Grenth – Wraith
Melandru – Drake
Balthazar – Gorilla
People in the map helped me figure out the way up — you want to stair-step up the updrafts on the -left- side of the falls as you face the falls. Problem is the updrafts aren’t permanent, and a couple of them visually blend in to the falling water, so unless you have a super good memory for the pattern you have to sort of luck into getting the up-boosts as needed.
Then the water has actual force and can knock you back off the ledge if you land in it, so watch out for that.
Once up there you go to the north end of the falls and jump up a semi-hidden path to find the spider’s den.
You can also glide around the cliff from the Siren training area. From the Siren training area go behind the choir, climb some boat wreckage, glide around the cliff to get on a sloped ledge, walk up to the highest part of that ledge you can, glide over to the next cliff, then glide around the edge to get to the top of waterfall area. You can fight the Unchained Spider or go get the mastery point.
Yet another way
After getting the mastery point over the waterfall, I got a ley-line organ and jumped over the wall to the North and landed on the roof of the pipe leading to the spider.
Aha! So, it’s basically just getting up to the MP ledge. Got it. Thanks.