Tom cannot catch a break. No sooner is he back from one adventure, then fate has changed him into one of Equestria's greatest war heroes. Thrust into battle against an ancient enemy, does he have what it takes to fight to the death in the hardest struggle he is yet to face?

Part of the Legends of Equestria continuity. Reading the other stories in the series first is strongly recommended, but at a bare minimum, you must have read Sands of Time in order to understand what is going on;

https://www.fimfiction.net/story/433091/legends-of-equestria-the-sands-of-time