Heart of War is a short story by Sarah Pine featured on the official site. It offers a glimpse into the experiences that have shaped Garrosh Hellscream into the leader he is today: his heroic exploits in Northrend; his uncertainty as he arrived in the imposing Horde capital of Orgrimmar; and most importantly, his friendship with his mentor, the warchief he would eventually replace, Thrall.
The following texts were not published as short stories but rather published on the official Blizzard websites as blog posts split into several parts. However, they are included here for completeness' sake.
World of Warcraft Community Site Heart of War