After defeating Amdusius in Maxos temple, a forcefield blocking off one of the passages there will drop (opposite direction from the chamber with the bedrooms upstairs). The teleporter at the end of that path will bring you to sentinel island.
If you are on the island, just keep exploring. If you don't find it before then, you will be led there when you are able to enter.
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.
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