Pretty fun old-school game. I actually thought the graphics were decent, and some nice design concepts were thrown in. The hero looked awesome. A few of the levels with "skull design" were nice, and I like the way you could see the stars peeking through the bottoms of the pits. I really liked the gateway on the final level.
Music is great old-school action/horror. The voice-sampling garbled, but a pleasant surprise!
First problem is that the weapon, as others have said, disintegrates too fast. Picking it up only to have it disappear just as I was reaching the monsters was frustrating. I understand you want to make the game challenging, but there are certainly other ways. Instead of turning invincible when you have the weapon, the game might have been better with a basic "hack through the enemies without getting hit" approach, maybe with the space bar as the attack button.
Also, I kept expecting to see more. Why only one type of weapon? Why only type of enemy? Constantly changing weapon and monster designs would help the keep the play interested from level-to-level (and you can still repeat designs when you have to). Some enemies could take more hits than others, some weapons could do different things. In fact, maybe the "invincible axe" we see on every level could be more like a special pick-up that only appears occasionally. And right now, the only level that was any challenge was the "dark level." If you added more levels, they could become more and more complex as mazes.
I think you have a good beginning. The game is quick and fun, but it has potential to be so much more.