After Desh'ea was written by Matthew Farrer and is a short story, just about 30 pages in kindle edition, that is weird in the sense that it's all about Angron's arrival at the War Hounds (that's pre-Angron World Eaters) fleet.
It can be found on its own as an audiobook at the Black Library audiobook or on Amazon, or in the Tales of Heresy anthology both in its Black Library eBook format or as a book on Amazon (with price tags depending on where you buy it; for example, it's cheaper on amazon.com than on amazon.es).
But, even if Tales of Heresy contains other interesting stories (for example, Blood Games for Custodes fans), I got it in another, much better anthology for World Eaters fans: Angron, from The Horus Heresy series, an eBook which can be found both on Amazon and on the Black Library. The reason is that in Angron* After Desh'ea comes alongside two other World Eaters-related short stories: Lord of the Red Sands and Butcher's Nails.
As I said above After Desh'ea is a bit weird. It only deals with Angron's immediate arrival to the Conqueror, the War Hounds legion's flagship. He's been teleported first from the field of battle (against Angron's will, as doing so left his comrades-in-arms alone facing the Nucerian high-riders who would crush them), to the Emperor's own ship in orbit over Nuceria and then, after a heated, bloody exchange with the Emperor and his custodians, to the Conqueror.
There he is met by the War Hounds master, and after the meeting ends unsatisfactorily, successively by other legionary commanders. It's only when Khârn, commander of the eighth (of course**) assault (again, of course***) company, meets him that a successful conversation is started, and Angron finally accepts to lead the War Hounds.
Khârn is soundly beaten all the while he tries to talk sense into Angron, which makes for, as said before, a weird scenario, but again we get samples that allow building a better understanding of Angron's frustration and understanding the space marines' longing for a leading figure.
*Angron is the title of this anthology, but also part of the title of some more stories, including two from the list I compiled about World Eaters, these being Angron: Slave of Nuceria and Angron: the Red Angel; when I get to comment these stories, I'll refer to them respectively as Slave of Nuceria and The Red Angel to avoid confusion.
**Eight being Khorne's sacred number, it makes sense as a kind of predestination quirk for Khârn.
***Khârn as a commander of a support company? No, it had to be an assault one.