The Problem of Flight
In the old days when magic was plentiful, flight posed an interesting challenge. Casting a spell that made the target float was kid’s stuff, creating a controlled flight harder, yet very possible. But as was discovered by several unlucky researchers, to fly as birds is much more difficult than suspected.
Magic is a strange essence, taking upon the characteristics of its surrounding. In the air, it becomes more and more air-like; and the higher one goes, the stronger become the currents of the supernatural element. As the winds and magics blast in the same or differing chaotic directions, the fine thread of a flying spell becomes unraveled, and the spellcaster has trouble to correct the problem (especially if falling with a high speed towards earth…). The occasional non-magical spots found up there do not help either.
Countless experiments were performed, with automatically re-casting and self-renewing spells, but all failed to produce anything close to a stable flight. Only after Archimos, now a largely forgotten sage and inventor (picture da Vinci with magic as one area of expertise) has tackled the problem, it came close to a solution.
Magic in itself was too unreliable, and difficult to convert to a permanently useful form. Technology, barring silly toys, was unable to hit the sky. (And it was proven that nonliving matter cannot fly anyway.) The logical answer was a fusion of technology - in the form of an artificial wing, and magic - in the form of a flying spell.
Every wooden beam of the Wings is composed of two parts, glued or otherwise joined together. Inside of them are bones from the wings of actual, flying birds. The bones must touch throughout the whole structure and be small enough - so common-sized birds are usually used, a few dozen of pigeons may be a good start. The skelet is wrapped into fine silk or similar strong fabric. Finally, at least the edges of the wings must be covered by feathers of those birds.
Much depends on the quality of the manufactured wing, and any damage may significantly weaken its performance.
This semi-magical item makes the cooperation of two worlds possible - the wings, while not large enough for flight, allow for gliding and can keep the height and direction for a while. Once proper rituals have been made, the structure easily accepts a spell of Flight, and fly it will, resisting to a degree the extreme conditions. Should the spell be disrupted completely, the Wings are immediately prepared to accept another. (It is also possible to ‘feed’ the spell continuously and thus to maintain it.)
The Wings are definitely not designed for a walk on the street, or to be collapsed in any way. Equipped with straps to hold the ‘wearer’, and implements to move the wings, a well-trained flyer can easily control it. (It should be noted that the wings are more movable when empowered with a spell, they seem to react instinctively on their own - apparently a memory of the birds that are part of it. The very wood bends slightly to the benefit of the Flight.)
None; as a semi-magical technology, the item is not magical itself, but requires magic to enhance its properties or make them possible in the first place. The spell of Flight in any of its variants will do; it may be necessary to recast it while in air.
Allowing unlimited flight can create issues even for worlds of high magic. (See an outside article for impact on cartography and warfare; bit on the D&D side, but makes a useful example.) This item straddles the border - it institutes problems that make magical flight difficult, and a possible solution. A Wing might have been a common item back then; now they are a legendary relic. And there is nothing simpler like making the few surviving specimens ‘ancient technology’, with the ritual needed long lost, or not even possible to perform; so every Wing that breaks is disabled permanently.
Oh, and yes: this thing requires much training to use. It’s too easy to kill yourself otherwise.