Alright, the possession starts from here:
http://www.onemanga.com/Katekyo_Hitman_Reborn/75/20/
if you read from here on it will make some sense, but it you want it to make a little more sense... read the past 1 or 2 chapters.... or read the whole thing because its all pretty good
Sorry if this has been found out before!!
SPOILERS kind of......
EDIT: I forgot something important... it starts as a M2F possession and then he takes control of multiple people at once.....