Moss Giant Island is a small island just off the coast of Karamja west of Brimhaven. Level 10 Agility is required to swing on the tree-hung rope to access this island. There are 5 Moss giants on the island, all level 42 and mostly aggressive. There are a few safespots as well, allowing for Ranged and Magic. Unlike other Moss giant spawns, this place is relatively vacant.
As the island is usually empty, players may find it a good place to train also as the giants are aggressive and there are some spots to stand where more than one will attack you without you having to move making it a good place to train attack or slayer. However, some players find that the numerous trees growing on the island can slow them down.
Curiously the agility icon does not appear for the swing to the island meaning that many people miss it.
As a side note, if you stand near the west side on high detail you will get a glimpse of an island. There is a single willow tree on it. This is probably the peninsula with the Necromancer's Tower.