SPOILER: In the game, attacking Dagon itself will not yield any results. You must continuously dispatch the 2x 'worshipers' that continuously respawn during the fight. Each one you kill reduces 'Dagon's health. (I think in the game they are actually 'priests').
(At one point, you may lose the ability to cast signs. This is frustrating, because you can't understand why this is the case: there's no apparent reason, no icon, no warning, no notice, just that the right-click button does nothing.)
You can, however, easily move around the small square of action without being hit. While you can, use any damaging signs you can the 2x worshipers; fire mostly, but the trap works well too. Dagon himself seems to heal the 'worshipers', but if you just keep attacking them, they will die eventually.
From memory, you need to kill 8 of the worshipers to kill Dagon. It may be 6x, or 10x, or even 12x... not sure. Just keep killing the "worshipers" until Dagon dies. Remember, you cannot attack Dagon directly and do any damage at all.
Unfortunately, if you're in 'advanced' vies (IE: not isometric), you may be VISIBLY clicking on a particular 'monster', but in reality you're attacking some other creature within range. VERY frustrating. If you're stuggling, try switching to isomectic view (F2/F3 I believe).
You shouldn't need 'blizzard', but I certainly needed to heal a few times, which requires White Honey to reduce toxicity to zero, then all the monster bits I could eat to get my vitality up again. [moved from the article page, original text added by Torrentcrazy, on 14:19, July 25, 2008 ] Game widow 18:25, 25 July 2008 (UTC)