one similar to the sword of Allah that Khalid ibn al-Walid (radiAllahu anhu) used. He was advised by Abu Bakr as-Siddiq (radiAllahu anhu) on how to handle the murtadeen and munafiqin in the war of apostasy.
He received instructions to execute them, thereby marking one of the most righteous actions made by the Sahabah. Beheading.
tbh imho though, i wouldnt use a katana nor buy one. i would buy a scimitar though. they seem rather useless for beheading, as you cannot get enough momentum with the point in which the blade will impact and sever the brain stem.
with a scimitar, a lot of the weight is in the very middle, and it is curved as to strike the neck with the most momentum filled part of the blade.
as for day to day combat, i still prefer a 7.62x39mm kalashnikov platform rifle. cant beat the classics.
katanas are rather outdated. have you thought about a scimitar/AK?