"With a daunting and unfamiliar name, the event called The Trials is actually a rather simple yet infrequent event in the War Dragons game. Think of this event as a very long boxing match, you against the world (Blackbloods world).

You will be able to use all your dragons starting with the lowest (red) tier first slowly unlocking better tiers until they are all unlocked. Yes, every dragon that you ever hatched can and should be used starting with the most noob on up to eventually your best dragons assuming they are in higher castle tiers.  As you use your dragons to face computer maps solo (no team help), your dragons will be beaten down. There is no heal time in the Trials, just like in real life boxing. Once your dragon's health is gone, it will not be revived in this event (it's virtually dead) unless you have coveted Miracles. Miracles which can be found sometimes in treasure chests will revive all your dragons one time. However, each arena (red, blue, etc.) has different numbers of required miracles to heal dragons. Outside the event, your dragons will function normally.

​HINT: have as many dragons at Expert level prior to entering this event. Health is key, so the more dragons with potential life left means the longer you can compete in this event. Hopefully you capped that Kinnara all that time ago!

You are competing against yourself basically, or other players if you want to think about event rank. Obviously players who have been in the game longer with more powerful dragons will have the best advantage in this event potentially lasting the longest through tougher and tougher bases."

http://www.wardragons.info/trials.html