An Invitation - Act 4 Side Quest

Start Location: Upper City of Lost Souls
Quest Location: The Salt Flats
Objective: Destroy the Daemon Carts in The Salt Flats

An Invitation is given to you by Callicrates, an NPC who is found in the Upper City of Lost Souls. To quickly get to this location take the Plains of Judgement portal and it'll put you right inside of this town. For the quest what you need to do is travel northeast to The Salt Flats and destroy 3 Daemon Carts which are parked relatively close to town.

Once you destroy one Daemon Cart a couple Machae Wardens will spawn and attack you. This only happens after you destroy the first cart, for the rest nothing will spawn. If you're having trouble locating the Daemon Carts take a look at my screenshot below.

Note: Completion of the An Invitation quest will lead to the followup An Inside Source. To begin An Inside Source just speak with Callicrates a second time after you turn in An Invitation.