Finding magical gear should always be a cool moment in a TTRPG, and yet, it seems to often be relegated to something that just adds a passive bonus to a few other numbers and that's it. That's boring. Here are four ideas for more interesting magic weapons for D&D and its derivatives (this is obviously not an exhaustive list by any stretch of the imagination; let it just serve as idea fodder to stimulate your own imaginations).
1. The Four Brass Nails
Each Brass Nail appears to be a brass stiletto with a ribbon of cloth tied to its quillon. In lieu of attacking, a character can stab one or more Brass Nails into a creature's shadow with a single attack roll against unarmored AC. On a hit, the Nail pins the shadow in place and exiles the victim's physical body into its shadow, protecting it from all direct harm.
Using a single Nail places the victim in a sort of suspended animation, negating the effects of hunger, aging, etc. Using two Nails causes the victim to maintain sensory awareness through its shadow and experience the passage of time. Using three Nails causes the victim to age naturally, potentially leading to death by old age. Using all four Nails allows the shadow to be moved by other creatures manipulating all of the Nails simultaneously without removing them from the shadow (i.e. it can be moved across surfaces but not through air).
The victim is released from its shadow when all pinning Nails are removed or if the environment is no longer able to create shadows (e.g. total darkness).
For best effect, introduce the item with one or two Nails having pinned a shadow in a place like the roof of a forgotten tower.
Inspiration: the ghost-trapping swords from Silent Hill 4: The Room
2. The Usurper's Axe
A demon-forged aged bronze battle axe, tarnished by scorches and verdigris except for its unstained cutting edge. If it hits an extra-dimensional target, they must make a saving throw (against spell, Will, mental attack, etc.) or be banished to their native reality. On an attack roll of 20, if the target is banished, the wielder also goes with them, leaving the axe behind.
Extra-dimensional creatures of chaotic nature are drawn to this weapon, for they have an innate understanding that it can be used to defeat their rivals in a single blow.
Inspiration: Auron's "Banishing Blade" overdrive from Final Fantasy X
3. Zxdes, the Vapor Blade
When not held, this appears to be a dagger hilt, glistening moistly. When held, a plume of wavering vapor emerges from the guard, gradually forming an undulating blade. This deals damage as a normal sword, and all attack rolls are made against unarmored AC as its intangible blade ignores the effects of armor, shields, thick hide, etc.
Zxdes is sentient and capable of communicating by imprinting colors on its wielder's mind (it does not understand that concepts like the sound of purple, smelling maroon, or CADMIUM YELLOW are nonsensical to most beings). Upon sensing combat, it extends cilia into the flesh of its wielder, making it impossible to release or to be disarmed. It deals no damage to unarmed opponents, as they are beneath its notice, nor to plants without ultramarine roots.
Lacking a physical blade, Zxdes is impossible to sheath, though the hilt may be carried safely if it is stowed without being gripped. When first held, it takes one round for the vapor to gather sufficiently to be used as a weapon. Once a creature has used it in combat, they will be compelled (as a permanent charm) to carry the weapon on their person at all times.
Inspiration: "Alien Vapor Kris Blade", a magical knife in an old MUD, along with Shieldbreaker from Fred Saberhagen's Swords of Power series
4. Makoto, the Gripless
Rather than having a normal grip, this saber's blade continues below the quillon. On an attack roll of 20, the blade slices through its target so effortlessly that the wielder makes another attack immediately (this can trigger multiple times consecutively). On an attack roll of 1, the bladed grip cuts into the wielder's hand, dealing damage as a normal sword.
Makoto is the host of a spirit which will use its telepathy to encourage the wielder towards melee violence. It can detect nearby weapons, all of which it hates passionately.
Inspiration: the two special katanas from Demon's Souls