How do I get keys to open Hatchery chests?

Keys (Dragonbone / Dragonglass / Dragonfire) mostly come from playing regularly:

• Active events (especially Hatchery chest events)
• Progression rewards (campaign, leaderboards, House Level)
• At the Market, trading your surplus Relics converted into Relic Shards

If you're out of keys, you can always open chests with gold.

Since keys are permanent (they don't reset), the smart play is to set them aside and spend them at the right moment — for instance when a "Hatchery Chests" event gives you bonus rewards per opening. Opening 30 chests during an event pays more than dribbling out one key at a time.