Why is 鬼蜘 pronuenced おにぐも and not おにくも?


I noticed that sometimes the pronunciation changes when combining words. An
other example is 不死斬り (ふしぎり) and 腹切り (はらきり); both 斬り and 切り are pronounced きり
independently but only in 不死斬り is 斬り pronounced ぎり and not きり.

Is there a rule for that?

My assumption is that it depends on the ending sound of the first word. If it
ends with a i sound (ni,shi,etc) then they lead to dakuten (") on the
following syllable.

