There is a Reaper Cloth here that you can reach by going down some stairs, going north, and surfing across the water. Give to to a Dusclops to make it evolve into Dusknoir when traded.
Use Waterfall here and you will find the sage Giallo. He will give you TM08 Bulk Up.
You can go west from here to reach Abundant Shrine, which is where you can catch Landorus if you have both Thundurus and Tornadus (you will have to trade one of those two into your game).