The Island was really small with the huge arena in the middle of it. Everyone could enter arena without any limits. On arena spawned all monsters: Ferumbras, Morgaroth, but also small monsters like energetic books from library.
There was also a mini museum with all bosses in it -
Ferumbras,
King Zelos,
Spectral Scum,
Hunter,
Demon,
The Last Lore Keeper,
Feverish Citizen,


The Scourge of Oblivion,
Killjoy,
Devovorga and
Gaz'Haragoth.
On the Island you were able to kill oldschool Bonelord
.
I was there for around 15 minutes and this is everything I saw there. I think the movie posted by Achiru shows almost everything. :)
There is a movie how this isle looks like - link