Defeating Amdusias will unseal the passageway (opposite the chamber with the bedrooms upstairs) that leads to the teleporter that will take you to Sentinel Island.
If you are in the main room at the start of Maxos temple, there are 3 tests that need to be completed in order to activate the teleporter there.
BTW, your first encounter on Sentinel Island you can mindread to go up a level, so the higher level you are (in D2:ED 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.
