The hallway in Maxos temple (opposite the chamber with the bedrooms upstairs) you can use to access that teleporter had been blocked by a visible force field until you defeated Amdusias.
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.
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