The Diamond Key

I killed the king, but I think the Priestess is a demon. Do you ever find out what is up with her?

to make the woman tell you about Kianmay, you need to be carrying the flute. To get the flute you need to get the Primordial Crystal from the Wolf, tell the Crow you are not a Champion of the Light, then choose the form of the Primordial Crystal such that it becomes the flute. I can't remember what choice yields the flute, but it should be obvious what it isn't.

In your final battle against King Darm, you can use various means to defeat him. The Big White Elephant represents the 'second best' ending in terms of finding a solution that doesn't involve just killing him. That said,it is probably the hardest ending to reach. The greatest flaw with The Diamond Key is that there are many non-essential sections that most people will never see because they are so hard to get into, yet they comprise many paragraphs. Were I to redesign it now (and if I wanted to make it really hard) I would put some essential element to victory in each of the hard-to-get-to areas. To find the Big White Elephant also involves a bit of luck, which I have considered asking Andy to change. Currently, in the palace when you search the throne room, 3 things can happen, or there is a 3 out of 6 chance of finding nothing. It would be better if one of these three things always happened. But as it is, you only have a 1 in 6 chance of finding the item you need to eventually meet the Big White Elephant. Finding the ending with the BWE isn't neccesarily more exciting than any other ending, so don't be too curious about it. To get there, I will tell you how to get over the biggest obstacle: When you leave Darken Wood, you must have gotten back your lockpicks (which of course you must have purchased previously in order to lose them). Once you have these, they will give you access to a hard-to-get-to area, where you can find a certain item. This item, when used with something you can find in the throneroom, will lead to a few endings involving the Big White Elephant.

This is a tricky one. If you lose the lockpicks in Darken Wood, then you won't have an opportunity to buy more until after the point where you really need them. When the Crow asks you if you are a Champion of the light, say 'no', then answer his question correctly about what the Diamond Key is a key to. As a clue, diamond is the most precious substance, and so the Diamond Key is the key to what is most precious thing in this world (according to the makers of the key).
If you can't figure this out, there is a way earlier to discover it. When you are on watch at night in Darken Wood, you have a 1 in 6 chance of observing a strange fire. If you go to the fire, go through it to hell, and defeat the demon; then upon your return, you will be told what the answer is.

King Darm. Especially any Skill upgrades

Firesword

But I still have problems with the Horned Monstrosity....I swear the dice have a vendetta against me

If you are fighting the Horned Monstrosity, it means you have failed to obtain any magical/trickery means to attack Darm. Try to obtain items such as scrolls and rings during the adventure, as these will give you more options at the end and you won't have to fight the Horned Monstrosity at all.

I'm not sure how to get both the the password Zytow zytox ha zytow zytox from Alyn Kyra *and* the Amethyst Condor Amulet from the Records Office, both of which (to my knowledge) are needed in order to retrieve the Revelation Scroll from the magician's quarters on the 2nd floor.

I can get either one of the above, but not both. As soon as I get one, I am forced to climb the stairs to the 2nd floor, and I can't get the other item. Am I missing something? Thanks.

The spell is not the only way to enter the magician's quarters. The door can also be opened with a golden ring that acts as a key. This ring is in the possession of the magician himself. You can find him somewhere in the city, pretending to be someone else. You just have to roll him and take it.