"Our fortunes rise and fall together."
Liaisons from other civilisations arrive with their caravans. If your liaison is killed somehow, he or she should be replaced next year and visit you eventually. The elevation of your colony will not occur unless the liaison leaves the map alive. You will, however, get another chance for elevation when the new one visits. It is speculated that donations to your homeland can speed this up.
Liaisons will attempt to attend a meeting with your leader. They will typically follow your leader around, waiting for an opportunity to discuss your situation. Your leader will need to perform the Conduct Meeting job, sometimes for quite a while, to satisfy the visiting liaison.
If your liaison goes without meeting for a long period of time (or your mayor was taken by a strange mood), he or she will leave and you will get a screen informing you "Diplomacy stymied: A diplomat has left unhappy". This also happens when the expedition leader/mayor is replaced.
The Dwarven liaison will seek to draw up a trade agreement. You'll be asked to indicate your demand for all the different kinds of goods; this will cause prices to be quoted, and will strongly influence the contents of next year's caravan. In return, the liaison will indicate the prices that will be paid for goods the dwarven civilization particularly desires (you are not obligated to trade any of those items). If you have satisfied the requirements for becoming a barony, the liaison will also grant you the opportunity to appoint a baron; upon satisfying the requirements for becoming a county or duchy, the liaison will also promote your baron to a count or duke. Dwarven liaisons no longer arrive once the monarch moves to your fortress.
In version 0.31.12 and earlier, Human and Elven caravans erroneously sent a liaison which claimed to be from the mountainhomes and happened to perform diplomatic functions; this has since been fixed.Bug:3089
In 0.28.181.40d and earlier, Human caravans included a merchant noble (typically a "Guild Representative") who would make trade agreements; however, they do not exist in the current version.