From the Mahou Sensei Negima! series.
Pactio is a magical item that is proof of a contract, depicting the Minister with their Artifact.
Posts having this tag shows a character as depicted in a Pactio card, is using a Pactio item, or is in one's Pactio form. If this tag is used in other anime or in other contexts, it is most likely a parody and should be tagged as such.