In order to enter Hellbound Island you have to complete Path to Hellbound quest. It's a complicated system within game where different parts and rewards are accessible according to the level of trust gained with Natives - might not be the same in all servers as it depends how many players go there and how fast certain tasks are completed. Trust points are accumulated per server and individual achievements are measured with the level of Caravan Certificate you have.
To enter Hellbound you need to port through the warpgate. There are 2 gates available - one in Heine and one in Wasteland near Gludio. When you use SoE to leave you will come out at the Wastelands gate though.
Quest rewards and NPC trade include S80 weapon and armor recipes, parts and materials. The mobs drop regular items like adena, materials and arrows but also things needed to raise the level of your Hellbound trust (to get access to additional features) and trade with NPC's. The specific drops include Darion Badges (tradeable), Ancient Tomes of the Demon (bound to the person who picks it up), Scorpion Poison Stingers (tradeable), Marks of Betrayal (from killing caravans, tradeable), Life Forces (from killing chimeras, tradeable). Most of the area is desert and hunting there at daytime (in-game time) gives you heat damage so you will need a potion you can buy from Hude once you have acquired necessary Caravan Certificate.
This map is made as of January 2010 on Devianne.
Note: Not all mobs are marked on the map to avoid too much clutter. There are more Junior Summoners and some other mobs around the area but those in the passage are not always there so don't be surprised if they respawn on top of you. Arcane Scouts need some extra attention since they can spot you even if you're using a shadow skill (Stealth, Dance of Shadows, Silent Move), most probably if you use invisibility talisman as well. Celtus is a Chimera mini-boss. Chimeras, scorpions and caravans are needed for various quest items so those are highlighted as well.