If Imu powers is from a devil fruit then Teach is the only one who has "the key" to defeat him
Thats why Teach spent 30 years searching for "the most evil devil fruit"
I believe Imu's ability is an ancient sorcery from the 1st world linked to the origin or tree of DFs and isn't a devil fruit by itself.
>Davy Jones fought Imu to standstill but still couldn't get past that sorcery and vice versa, prompting Imu to seal him in the depths of the sea.
>It's there where/when Jones wished he could have the power to kill Imcel
>Due to Jones wish being the ability to outdo the very magic that spawned the fruits, it was manifested into the devil fruit that is Yami.
>Gives a whole new meaning to Teach's most famous phrase "A Man's/People's Dreams Never End", it actually references the biggest plot of the series.
Yami is said to be the rarest Logia, and that by default makes it the most valuable devil fruit ever.