The Forbidding Island is an island where the quest [32] Waking a Demon takes place. It can only be accessed by ship from Illaneska by speaking with Centurion Cadia.
Group Summon will not work here.
The Island[]
There are four Tribal Demon Generators on the island. Each of them is guarded by a level 32 Demonic Keeper. They don't respawn.
You need to deactivate all of the generators to complete stage 2 of the [32] Waking a Demon quest.
Resources[]
Inside the Crater[]
Inside the Forbidding Island Crater there is only the level 33 incarnation of Gamacaris.
You need to scare him away to complete stage 3 of the [32] Waking a Demon quest (he will flee when his HP is down to about 25%.)