Hmm... Ulmo and Desi can't see each other now, due to the acid fog I believe, so Ulmo only knows that his current opponent is not in the open area and not in the VZ to the west of the western wall of "the box".
If those assumptions are wrong, and Ulmo can see Desi, then I'll need to know what Ulmo knows about Desi instead of doing this:
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Uses the more powerful of his Control Water spells to raise the water level then fly to 105n65e40z and land on the roof of the VZ.
Is it acceptable to think that from standing on the VZ roof he can crane that long neck over the edge to look into the VZ below? Somewhere there must be rules about peeking around corners.
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