The Dead Wars were a series of conflicts between the Ytarrans and the forces of various Vampire Kings, instigated by Ytarran expeditions into the Shadowfel. After swiftly routing the Ytarrans from the Shadowfel, Hastings counter-invaded into the Fadelands and was soon joined by fellow Vampire King Vladimir, who targeted Ytarran facilities in the North. Eventually, after Alyssa joined in as well, the Kineticists' Guild (and possibly also the rest of the Adar - the records are a little unclear) joined the war, helping to turn the tide. Also (timing relative to Kineticists' intervention uncertain), the Ytarrans invented the Warforged, who as well as being stronger and more durable than Ytarrans were immune to many of the necrotic effects employed by the Vampire Kings and their lieutenants.
Eventually, a peace accord was reached whereby the Vampire Kings would stay out of the Fadelands as long as the Ytarrans (and presumably also Adar) stayed out of the Shadowfel. As both Swayne and Cornello have ordered incursions into the Fadelands of late, it seems that they take deals with the Ytarrans to have died with them. Alternatively, it is also possible that Swayne simply did not take part in the wars in the first place (as his territory corresponds to Vistria, which had few if any Ytarran facilities) and it is unknown as to what extent Cornello feels bound by his predecessor's treaties.