I've placed an enchantment on the tabard that should allow you to slip past the Sunreaver's Sanctuary's defenses for a short period of time. You should have enough time to find your way into The Filthy Animal and meet up with the Sunreaver contact there, one Magister Hathorel.
Obtain the copy of Ancient Dragonforged Blades that his men borrowed from Wyrmrest while your disguise holds, and return it to me.
The disguise (buff) will last 5 minutes. This may be the one opportunity that Alliance characters have of seeing the interior of The Filthy Animal.
Jump down to street level, mount up, run across Dalaran to Sunreaver's Sanctuary. Take a right at the fork. The Filthy Animal is on the left. Run straight in towards the back of the room to find the only neutral NPC.
Magister Hathorel: At last, you're here. We have to get this tome to Icecrown as quickly as we can.
I'm ready to deliver the tome, Magister Hathorel
Magister Hathorel says: You're late, courier. No, I don't want any excuses; this errand is far too important.
Magister Hathorel says: Here's the tome our representative brought from Wyrmrest. Get this to Myralion Sunblaze immediately.
Magister Hathorel says: Watch your back, courier. I needn't remind you of the value of the book, nor the fact that the Silver Covenant will stop at nothing to take it from us.
If, for some reason, you need another refresh of the tabard, speak to Tybalin again: