Unseeing Eye quest - question and spoils

Im wondering about the unseeing eye quest in the Temple district. I've cleaned out the sewers and met up with Mekrath, retrieved the silly mirror, and then killed him (he was evil, I'm a paladin). However, I cant figure out where to go from here. I see the other secret door in the sewers, but I cant open the stupid thing. I tried to force it - no go. Can someone nudge me in the right direction? Is it possible to open the other door? If so, how?

btw.. I noticed that when I decided to kill Mekrath, he was able to get off a ton of spells before finally going down. Even when I cast silence on him! I know he has Contingency set up, but that is ridiculous! I also noticed that he casts Raise/Summon Undead, and produces 1 or 2 skeletons. Well... If my main character is the room at the same time as he casts Raise/Summon Undead, I DIE INSTANTLY! What the hell is that? Someone trying to tell me something? And.. anyone know what level he is?

[Yorick]

The key to that door is nowhere nearby. Forget about it for now, for you have to practically complete Lord Jierdan Firkraags mission to get it. He's in the copper coronet. Actually, if you're a Paladin have you not done his quest?

I can spoil if you wish, otherwise keep going.