What if Yami never existed before and Xebec was referring to Nika and a different fruit? What if it was the manifestation of Xebec's dream of something that can harm Imu, and it spawned following his death and was formally added to the DF encyclopedia years later.
And the reason why Imcel keeps himself concealed is because he's afraid someone would recognize his powerlevel and wish if he could defeat him, only to get his wish manifested later in form of a fruit.