The key it gives you seems to be random or may depend on which keys you already have. You may want kill the Magis many times to get all the keys.
At any rate, grab the key, and notice which type it is; it's going to tell you which door you can open.
11) Head back to the n,e,s,u,d room and start searching around, first try
going all northish/upish until you hit a door. If the door matches your key, you can open it.. if not, head back start searching around for the other doors (try going n or e, then all upish or head back to the eaststairs
and search around there)
12) At any rate, you will find the door for the respective key eventually and enter. You will encounter one of the 3 Torocks and a Magical Orb. Try asking the Torocks for clues about the chalice or quest and watch the reaction. If you find Torock the Fallen (crystal key), you can kill him and loot a gold cup! Congrats! You have found the chalice! Try dropping the cup for fun. :) You can also check out Torock the Holy (quartz key) and Torock the Gray (glass key).
13) Now, remember that bookshelf? Head back and push it to return to the blessing priest. After that, head e and n to find the
Holy Altar. This is where you have to place the cup on the altar but first look at the cup notice that it's empty. Go Look at the Entrance Door and i'm sure it will be quite clear what you need to fill it with.
So fill it with the dark and light (sun spirit and moon spirit) then just go back and place it on the altar. And you get the golden whistle GOOD JOB.