The Battle-Tested Guardian
The Battle-Tested Guardian is a quest for the Royal Ancient Lab found in Age of Calamity. It is unlocked after completing the prior quests The Ultimate Relic & The Relic of Legend, and requires the Challenge EX Alert: Forgotten Temple to be beaten as part of its completion conditions.
Completing the quest will unlock the Battle-Tested Guardian as a playable character; as well as the Battle-Tested Guardian quests EX The Final Battle, EX Regular Maintenance, EX Battle-Tested Guardian Training, EX Battle-Tested Research, EX Lessons of the Past, and EX The More the Merrier.
"At last! We've gathered the information we need, and now we now have a powerful new ally... the Battle-Tested Guardian! The ultimate weapon to protect Hyrule. The time has come. Now go greet it!"
"It seems the Guardian has agreed to fight on our side. I can't begin to understand its motivations. Then again, it WAS built to protect Hyrule, so perhaps it makes some kind of sense."
- Complete EX Alert: Forgotten Temple