Iax's climate and fertility have made it one of the most naturally agriculturally productive worlds in the Imperium. The inhabitants have harnessed the planet's inherent fertility, covering its surface with well-ordered farms and cultivated woodlands. There are no large cities on Iax, but many small towns dotted over the landscape, connected by an efficient system of fast hydroways. The oldest and most densely urbanised area of Iax is the ancient fortress city of First Landing, whose tall citadel has withstood the barrage of invaders over the centuries.
At the time of the Horus Heresy in the early 31st Millennium, Iax was one of the four Master Worlds of Ultramar. These worlds were the seats of government of the four major fiefdoms of the Five Hundred Worlds, ruled over by an official known as a Tetrarch. These "Princes of Ultramar" were highly distinguished Veterans of the Ultramarines Legion, second only to the mighty Guilliman himself and providing him with a chosen champion and an Honour Guard. The Tetrarch of Iax at the time of the Heresy was Stolos Amyntas.
- Codex: Ultramarines (2nd Edition), pp. 14-15
- The Horus Heresy - Book Five: Tempest (Forge World Series) by Alan Bligh pg. 81
- Know no Fear (Novel) by Dan Abnett