The origin of Glasstouch Daggers is unclear. Though several have been found over the centuries, hidden in dark dungeons and deep caves, none have borne the identifying mark of any known craftsperson. Study of the magics used in their creation show that they must have been crafted by a powerful mortal illusionist, rather than coming from any otherworldly source. Regardless of their origin these daggers are a boon to any who find them.
Though they are made of glass, these weapons are not fragile. In fact they are very nearly impossible to damage without going to extreme lengths to do so. Even the edge of the blade does not dull. Aside from this small boon, however, the Glasstouch Dagger functions in combat just as any other dagger would. The true magical value of the dagger lies in the magical feats it can perform.
When the tip of the blade is tapped against any non-living surface, that surface becomes completely transparent. This effect functions on an area of up to 10 cubic feet, though if the object touched is smaller (such as a door) then only that object will be affected. The surface does not become fragile, nor does it lose any of its other properties. The dagger can affect an unlimited number of objects, and those objects will remain transparent for as long as the dagger (or the dagger wielder) is within line-of-sight of the affected object. Remember that transparency works both ways. If you use the dagger to look through a door, then whatever is on the other side of that door can see you too!
If the pommel is used instead of the tip, then the surface tapped will become mirrored rather than transparent. This effect is subject to the same limitations as the transparency effect, though it only affects a single side of the object. If the pommel is tapped against a door, then the side it was tapped against will become mirrored, while the opposite side of the door will retain its normal appearance.
Any sage with basic knowledge of magical items will likely know the above information. However, few sages know that Glasstouch Daggers actually possess even greater magics which only those who are attuned to them can access. After a character gains 1 level with the Glasstouch Dagger in their possession, it will become attuned to that character. The next time they handle the weapon, it will feel vaguely warm to the touch for a moment, and they will have an indistinct sense that something about the dagger is different. A sage with detailed knowledge of magic weapons may be able to inform the character of what has occurred. Otherwise the below must be learned through experimentation, or divination spells such as Identify.
If the tip of the blade is touched to a surface, and used to trace a shape, then only the space within that shape will become transparent. This still only functions to a maximum of 10ft in any direction. Tracing a shape with the hilt produces mirrored surfaces in the same way.
Additionally, if an attuned wielder can stab the dagger into a non-living object, then that object will become as brittle as glass for as long as the dagger is embedded in it. Note that embedding the dagger in an object does not make it mirrored or transparent. It retains its normal appearance, but becomes brittle and easily broken. Note also that this ability does not allow the wielder to stab the dagger into an object which they normally could not. A stone, or suit of armor, or shield cannot be affected by this ability. However, a wooden tree could be.