What Can You Do On Mage Island?

The Seaskipper talked about a demon living on Mage island when i first sailed there, so I came with my Lvl 105 Mage to investigate. I found a tower at the end of the island, got to the bottom floor and found nothing but an empty room...

... at least until I heard some noises and saw Enderman particle effects, along with an Enderman appearing every now and then for a brief second.

Is there another floor I don't have access to yet?

 

I've never heard the Seaskipper say this, but in Grookwarts (do Order Of The Grook) there's a large door that can be opened revealing a mob in a cage and some lore surrounding it