The High King of the West, known also as the Leader of the Free Kingdoms but generally referred to simply as the High King, is a martial post invented and first held by the Legatus Praetor Dwail of Rigdon, known to the people of Engelain today simply as Dwail Shadowfoot. The title was originally created as an opposition to the Emperor in Messia, when the secessionist war of Westen Engelain against the might of the Tyrannian Empire was first launched. Since then the title has been worn only two more times, and in all three occasions this was in order to fight back against the Empire.
The title of High King can, in theory, be given at any time, though in practise the High King is only elected when war is declared with the Tyrannian Empire and hasn't been used in hundreds of years. Whilst Dwail Shadowfoot was self-crowned, and legitimacy later given by those who came under his banner, a High King can now only assume the post at the kings moot. The kings moot is a gathering of the thirteen kings of the thirteen Free Kingdoms, where through a majority vote, (seven backers,) one of their own is ascended to the rank of High King.
The High King of the West has absolute authority over all of the Free Kingdoms for as long as they hold that post- which has historically been for as long as the war with the Tyrannian Empire at that time. The High King leads all of the armies of the west, and his decisions supersedes that of all of his fellow kings. When the war has concluded, it is customary for the high king to resign their post and give back sovereignty to the individual kingdoms, as was decided at the first kings moot when Dwail Shadowfoot resigned from his post as High King.
The second instance of the High King being chosen was roughly a century after the first. The formation of the Free Kingdoms was wrought in blood, for after the forced departure of the Empire and within a generation of their foundation, the Free Kingdoms were engaged in a long and bloody war known simply as the Seven-Decades-Slaughter. Seeing the Free Kingdoms as weak and divided, the Tyrannian Empire sought to regain control of the land. A kings moot was immediately called, and the warring between the nations was forestalled in order to tackle this exterior threat. As such, the Empire in fact resulted in the end of bloodshed within the Free Kingdoms and a greater unity between them, solidifying their right to rule themselves indefinitely.