To do "Prison Break In" you need to kill the thre non-agressive undead NPCs inside the main area, then one more on the top floor of the large building outside of the camp.

Afterwards, turn them both in at tarren mill and get their respective follow ups, then go on to step 2.

Both of these quests are done in this large area around Dalaran.

After finishing both quests, you can either run back to tarren mill or die on purpose which puts you at the tarren mill graveyard. Either way, turn in both quests and then go to Undercity and look for a yellow + sign note which will inform you of what to do next.