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Did you go downstairs from the room where you fought the shadow lich? You should have gotten to a temple and found half an artifact rod (after some pretty tought battles against some eye tyrants). Then you should go to the bottom rightmost corner of the "inside-stomach map" as you call it
and you'll find the other part of the rod there. You can then use it to defeat the Unseeing Eye. The battle will be VERY VERY difficult if you don't have this.
If you're having trouble beating the normal beholders, then maybe you need to try this again after you advance a few levels. There's an exit to the surface in the bottom middle of that "inside-stomach map".
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