What resets at the end of a season (Reign) in Dragonfire?

At the end of each Reign, all your "disposable" development goes back to zero. What resets:

• Your dragons' level (Reign Level)
• All stronghold buildings and their levels
• All research (Infrastructure, War Tactics, Dragon Care)
• Your base resources: Wood, Stone, Iron, Food, Meat, Resolve
• Tile ownership and Territory Influence
• Your House Level (which decides which Heirloom bonuses are active)

In short: your physical empire (territory, buildings, army levels) is wiped every season. That's by design — each Reign is a new race on a new map. Don't stress about "losing" those things: everyone restarts equal, and your real capital (see the article on what stays) is preserved.