At the worldbuilding panel at PAX West this year, the Magic team showed off some of their images from the Ixalan world building guide.
Here are these images, for you to peruse and learn more about the Sun Empire, the River Heralds, and the Legion of Dusk. You can see larger versions of the images by clicking on them.
The Legion of dusk are vampires following a strict church from the continent of Torazon, coming to Ixalan by ship to find the Immortal Sun artifact that was lost to them hundreds of years ago.
The River heralds concept art.
The Sun Empire's concept art, as well as that of Huatli.
You can find more on the plane of Ixalan on the main world guide article.