Rites of Ascension
Twilight shared a quick look with Spike, a wordless request between two who have shared most of their lives with each other. Translated, the message was simple: "He's yours. Sic him." In response, Spike turned around and blocked the path of the incessant assistant of Duke Esteem like a boulder in front of a doggie door.
Rites of Ascension - Chapter 55: Flying On Hoof
Twilight starts a trek to the core of Cloudsdale.