Everyone knows that hell is going to be a separate existence in the minecraft world, this however raises the question of entering and changing existences. I think that it should be some sort of a tiny cave area found near bedrock, and inside that area... you find a gate. Gates would be demonic horrors made of screaming bodies that allow travel between worlds. If you break into the correct cave, you will hear whispers, if you see the gate, this music begins while you hear the screams of those that form the gate:
If you look at the gate, you get a sort of bloom effect where everything around you darken, the gate brightens, and veins start to grow onto the screen:
This would terrify ANYONE and would most likely make everyone worship the gate and never enter it.
If you want disturbing screams when you go through the gate try using the noises from the smoke monsters in the hl2 mod korsakovia. and I think adding those specific screams to minecraft would be awesome/terrifying at least while going through the hellgate. but i think it should be craftable along with being found naturally.