Nether-Stalker Mah'duun wants you to destroy 5 Tempest-Forge Destroyers. Return to him in Shattrath's Lower City once they all lie dead in order to collect the bounty.
There are mechanical constructs within the Mechanar which Mah'duun has been told must be dismantled.
Go to Tempest Keep, find the tempest-forge destroyers and destroy them.
Then you may return to collect the bounty.