I suspect this Imsama is ether a woman (no king can sit on the throne but they never mention a queen
) and I think there are more people like imsama. Man this chapter is insane.
Just hit me that theres another possibility
Imsama could be a spiritual manifestation/spirit avatars of something like say the DF tree, will of D. Etc or something grand
Spiritual manifestors in OP take the form of all black silhouettes as we have seen with the Kblauterman(Water spirit/fairy) and Going Merry(Skypiea arc when it was reparing itself)
Perhaps Imsama is a spiritual manifestation of something, which could explain is all black silhouettes like appearance
I also agree it's likely there could or could have been more like him
OP is a "anything can happen", "Endless possibilities" series and universe
so really who knows what he is, perhaps something none of us would guess