How to Unlock the Last Boss in "The Steamvaults"by Jess Kroll ; Updated September 22, 2017
"The Steamvault" is the third and most difficult non-raid instance in the "Coilfang Reservoir" released as part of the first "World of Warcraft" expansion, "The Burning Crusade." As with all "Burning Crusade" instances, "The Steamvault" can be played on either regular or heroic mode. Both modes are tuned to level 70 characters, with heroic requiring a higher gear level to complete. The instance contains three bosses, with the last of the three having to be unlocked after killing the previous two.
Defeat Hydromancer Thespia. The first boss of the instance is found toward the north of the first area. Consult the map of the instance (by default, pressing "M" brings up the instance map). Run up the low ramp toward the Hydromancer and her two elemental guardians. Once she is down, activate the control panel behind her, which resembles an organ.
Defeat Megkineer Steamrigger. The boss is located in the south of the instance. Run up the large ramp in the first area, leading to a different one. Take the right fork and continue south. Progress down the path occupied by Naga and Broken until you reach the large area occupied by Leper Gnomes. Steamrigger is located behind a couple of Leper Gnome packs, surrounded by a group of his own. The Gnomes are easily beaten with area of effect spells. Once Steamrigger is down, activate the control panel located behind where he stood.
Walk back into the Naga section of the instance. When the path to Megkineer Steamrigger ends, move to the right, continuing the path of the ramp from the first area. The door to the final boss is unlocked. Inside the chamber are several Naga, as well as the final boss, Warlord Kalithresh.