When you complete Maxos temple, one of the hallways that was blocked off on that level by a force field gets opened, which leads to a teleporter that brings you to Sentinel Island. There are a few quests there to do and areas to explore, before you can take over the battle tower and get the ability to transform into a dragon. You'll want to pretty much finish off Broken Valley and Sentinel Island before doing that, though (there is one quest in each location that can be done later).
If you made it to the island, but got stuck at knowing what to do next, a hint that may be appropriate:
You got a book of magic that can control wind when you rescued the guy on the top of the pillar (near the trainer Kenneth). There is a windy spot near where you start on the island with a tombstone and three candles...
BTW, your first encounter on Sentinel Island you can mindread to go up a level, so the higher level you are (I made it to level 18 in the fjords) the more of an experience bonus that will be. You don't need to get to a high level, but it would be better if you got to a new level before entering, to get a full level bonus.