If you have a copy of the Dungeon Master's guide, look towards the front of the magic item section, there's a few pages about personalising magic items that can give them additional minor bonuses.

I did it for one of my characters recently, Use the charts to determine creator, who it was made for, some minor anomalous (Beneficial, flavour or minor inconvenience) properties it has then you could use the minor beneficial and detrimental charts in the artefact section and give it one of each to produce a unique weapon, such as this one below I rolled up for one of my groups characters:

Katana was forged by a dwarf. The Blade is very durable for it's thickness and bears runes that extend 1/3rd of the way up the blade.
An old legend tells of an ancient hero that once wielded a weapon that appeared much like this one, drawing you to it. however, it's cursed nature was revealed to you once you attuned to it, as the blade regularly peals for offerings of fresh humanoid blood, or the weapons properties cease to function.
The weapon, when placated, whispers warning to you when an enemy is near or plans to strike, you get +2 to initiative rolls as long as you are not incapacitated
the blade can read, speak and understand the languages you can understand.
your blade can "hear and see" out to 30ft
the Blade, when placated, is of good temperament and of a neutral good alignment. but when not anointed with blood, it quickly becomes temperamental and aggressive.
The blade also has an agenda of it's own, seeking to lead a bloody crusade against the servants of a particular Deity, seeking to defeat, weaken or destroy the deity's servants wherever they may dare take root
Whilst wielding the blade, you have resistance against slashing damage. however, animals are instinctively aware of the blade's bloodthirsty nature, leading animals to react with hostility to you when you are within 30ft.

And an angelic one that happened to be rolled that night:

weapon was created by celestials, the weapon is half normal weight and inscribed with feathered wings, suns and other symbols of good. fiends find the item repulsive.
The weapon is central to an ancient prophecy and it's bearer is destined to play a key role in the unravelling of the event.
the bearer can understand and speak Celestial.
Whilst being wielded, you feel fortunate and optimistic about the future. Butterflies and other animals frolic in the items presence.
can cast Sacred flame as an action.
Your appearance takes on an angelic twist (Faint halo and transparent shining wings).