Odinani divides the work of justice between two deities, and the division is precise. Ala owns the ground and judges what the community can see: the abomination that pollutes the land, dealt with by public cleansing in front of everyone. Amadioha handles what nobody saw. The theft with no witness, the oath sworn falsely, the poisoning attributed to no one, the harm done in private, are all understood to remain within his reach after every human means of settling them has failed.
The instrument is lightning. A person struck by lightning was, in the older practice recorded across a number of Igbo communities, understood to have been judged, and the death carried a stigma: the body might be denied ordinary burial rites, and the site of the strike treated as requiring ritual attention. This is not merely a symbolic association. It is a working legal fiction with real consequences for a family, and its severity is why swearing by Amadioha was a serious matter rather than a form of words.
The oath is where he enters daily life. Where a dispute could not be settled by evidence, the parties could be required to swear, and swearing by Amadioha meant accepting the strike as the penalty for lying. What made the mechanism effective was not that lightning reliably followed, but that people believed it might, and that a person confident of their own guilt would often rather settle than swear. Colonial officers and later courts displaced much of this, and the oath's standing declined with the ritual authority behind it, though the association of a lightning death with judgement outlived the institution that used it.