[Terenas] Karazhan - Where are your upgrades from

[Shamago]

Farm Bosses (O = Optional for progression as best as I can tell)
Attumen (O)
Moroes
Maiden (O)
Opera Event (Romulo & Julianne, The Crone (Oz), The Big Bad Wolf)
Curator
Chess Event (O) (Dust Covered Chest)
Shade of Aran
Prince Malchezaar
Netherspite (O)
Nightbane (O)
Illhoof (O)

Chess Event is not optional if you want to get to Prince, and Shade of Aran is.
It is highly recommended that you kill Aran, as not only does he control ~60% of the trash in the path between Curator and Prince (~35% Netherspite 5% Illhoof), but you can teleport to his room from the doorman, which helps a great deal with time. You also have to kill Aran as part of the chain to summon/kill Nightbane.

I've also updated my list.