Legend of Carp Becoming a Dragon

A Zen legend known as Koi-no-Takinobori in Japan, wherein carp swim, against all odds, up a waterfall known as the “Dragon Gate” at the headwaters of China’s Yellow River.  The few successful carp by turn into dragons. 
The story symbolizes
the virtues of courage, effort, and perseverance, which correspond to the nearly impossible struggle of humans to attain Buddhahood. 

The eastern gate in a temple is also called the Dragon Gate, the most important gate on the grounds, formal ritual instalment of an abbot the most important event in a temple's ritual takes place at the Eastern Gate. Entering through the Dragon gate is also another phase for Awakening.