pretty low chance, 20%.. I just buy a kuva lich or sister that has the one i want.

Twice as much as dropping something from a rare relic, so yeah, 20%.

looks like your hunt for that ephemera is like my hunt for the magnetic one. took me 6 dead liches until i got it finally.
But look on the bright side: if you do not really care about the gun it has, the more you try getting it the better the kuva guns will become from the valence fusion.