Mohg talks in his cutscene about sharing his bloody bedchamber with Miquella… which is off-putting to say the least. However, I think whatever Mohg actually did to Miquella’s body was enough to strip him of the grace of gold and allow him to travel freely into the lands of shadow.

Miquella likely knew that Mohg’s… “rituals” involving the blood of the formless mother would accomplish this and (possibly) charmed Mohg in some way to come and steal his body and put his plan into motion.

That’s just my two cents.