This achievement requires the player to occupy four specific cities. It can only be obtained in the Eye of the Vortex campaign (as of writing) as the Mortal Empires map does not contain Thrice-Cursed Peak.
The four cities that need occupation are:
Thrice-Cursed Peak, in the Southern Spine of Sotek region, part of Lustria (occupied by the Spine of Sotek Dwarfs faction)
Vaul's Anvil, in the Caledor region, part of Ulthuan (occupied by the high elf faction Caledor)
Titan Peak, in the Ashen Coast region, part of Naggaroth (not occupied at the beginning, but near the Exiles of Nehek)
Star Tower, in the Volcanic Islands region, part of Lustria (occupied by Clan Skryre or a minor faction (not sure which))
When all four cities are occupied at the same time, the achievement should unlock.