Upon your arrival you'll discover that Leah was imprisoned during her journey to Alcarnus. Speak with Tyrael to begin this new mission.
Talk to Asheara at the Gates of Caldeum
Travel to the Caldeum Bazaar where you'll have to talk to Asheara. She can be found in front of the palace and once you're done talking you'll be allowed to continue the quest.
Talk to Asheara to enter the Imperial Palace
Follow Asheara to the Gates of Caldeum. Stop in front of the large gate, initiate a conversation and then enter the Imperial Palace.
Talk to Imperor Hakan II
A short cut-scene will show the meeting with the emperor and you'll also meet with Leah.
Kill the demons in the Imperial Palace
The guards will soon transform into Writhing Deceivers and you've probably met similar creatures before. Thankfully you can count on Leah's assistance during this battle and you'll have a lot of room for manuever. Don't forget, however, that the monsters are masters of illusion and therefore can approach you without being seen.
Escape the Imperial Palace
Once you've killed all the monsters the south-western gate should open and naturally you must travel there (Leah will join you again). Enter the City of Caldeum and stay on the wide path. You'll be stopped by new groups of Writhing Deceivers, as well as by Scaled Magi (they can cast lightning spells) and by Imperial Guards (they prefer melee combat).
Demon Summoners destroyed (4)
The demons are being called by four Demon Summoners, so while fighting the beasts you must also remember about destroying the summoners sooner later than later, preventing more monsters from showing up.
Follow Leah to the Sewers of Caldeum
Once you've destroyed all Demon Summoners you will be allowed to resume your journey. Keep following Leah until she'll locate an entrance to the sewers. Go there to end this quest.