Secluded from the mainland, Arkham Island was originally owned by the Arkham Family, and only contained the Arkham Mansion and the Botanical Gardens. Over the years, that further developed into becoming The Elizabeth Arkham Asylum for the Criminally Insane - an institution that was founded by Amadeus Arkham, and was named after his mother whom he euthanized here as a treatment for her dementia. Throughout the island, there are many little easter eggs, as well as Riddler Trophies, and tapes. Arkham Island is often replenished with goons for batman to fight. Also, you can hear many messages from joker through the com system.
A graveyard can be found in the part of Arkham where the Arkham Mansion and Botanical Gardens. There, Amadeus Arkham's grave can be found.
Also, there can be many bodies found, from both inmates as well as guards.