Eureka Key FFIII Art
The key that opens the way to Eureka.

The Eureka Key (エウレカの鍵, Eureka no Kagi?, lit. Eureka's Key), also known as EurekaKey, is a key item in Final Fantasy III, which is required to open the way to Eureka, an area located within the Crystal Tower that houses the most powerful weapons. It can be obtained by defeating Doga and Unei at Doga's Grotto