Speak with Shadow Priest Sarvis.
About time you woke up. We were ready to toss you into the fire with the others, but it looks like you made it.
Speak with Shadow Priest Sarvis in the chapel at the base of the hill, he will tell you more of what you must know.
To get out of this crypt, take the stairs up to the surface. Follow the road down to the chapel to find Sarvis.
From the Shadow Grave crypt, run north down the path towards the part of Deathknell controlled by the Forsaken. The chapel is the first building on the left. Shadow Priest Sarvis is in the back of the chapel.
If you do not accept the quest and talk to Shadow Priest Sarvis and accept the quest  , you can't get this quest from Undertaker Mordo. You will have to abandon all other quests you have to access this one. If you complete any other quests, you will not be able to get this one.
Another of the walking dead, hm? Must have been quite a shock, waking up in the crypt with only the cold and Mordo to greet you...
I see the confusion on your face. Let me try to explain our... situation... to you.
We have been freed from the control of the Lich King by our new leader, Lady Sylvanas. The Dark Lady guides us in our war against the hated Scourge and the holdouts of humanity who dog our every step.