I made it to the Black Gnarl but apparently I missed an item required to make the coffer / container. However, I cannot seem to go back. Leaving Fire Mountain, there is only one way to move, and you can only go forward one step. Same with leaving the Dark Tower--there is the way to the cliff (where you arrived), and the way that connects with Fire Mountain. I've tried using the "Whistle of Fiery Summons" from outside Dark Tower, both on the arrival side and the Fire Mountain side, but it says that the whistle does nothing. Any ideas?

NM, I found the answer. There is a blue portal thingy off the entrance room to Dark Lord Tower. I'd found it before but hadn't labelled it on the map and then put the game down for a bit and had forgotten about it--time to label that puppy.