The Blade of Legions:
When activated, the Blade of Legions teleports the wielder to a foe in short range, and creates mirror images of the wielder at other foes as well. A successful hit (not damage) by either the wielder or a foe against the wielder (not an image) causes the wielder to be teleported to another random foe. An image replaces the wielder at the foe just left, unless an image is already there.
The basic idea is that a person wielding the blade is a threat everywhere. An enemy can't be sure that the person in front of them isn't an image, especially since it might be an image now, but 10 seconds from now it could be real.
While a single blade like this is certainly of use to a rogue character, it becomes nastier when paired with multiple people wielding a Blade of Legions. There's a certain threshold though, as any hit causing the wielder to teleport means that while a single wielder can damage folks, if there's too many foes and the wielder isn't high enough level in comparison to them, all it does is become an annoyance/delaying tactic. Not enough damage is actually being dealt by the wielder in a single instance to significantly influence things.
The ideal mix for a wielder of a Blade of Legions is probably going to be 3-4 foes for each wielder. This allows for them to potentially issue damage that's meaningful, while maintaining enough uncertainty as to which wielder is actually there and which is just an image.
This is an item that PCs could certainly benefit from, but would also be nasty if a pair or small team of assassins (depending on the size of the group) were equipped with.
Hammer of Weight
It's not really known what exactly the origins of this "weapon" are; many suspect a Trickster to be responsible for it, while others claim a dwarf or gnome with a sense of humor. Regardless, the operation of the weapon is simple. Everytime an item is struck, the mass of it doubles for 30 seconds. The hammer itself is completely unremarkable, except for appearing to be slightly small for a weapon.
It's unusual and of dubious utility. It's going to be an especial temptation to people (NPCs or players) that can't fathom putting effort into creating a magic item that is more problematical than beneficial.