In our interview with Zedeck Siew, he said some beautiful, evocative, eloquent things about how there are "other ways to be" beyond the rigid strictures portrayed in classical heroic literature (such as the Hikayat Hang Tuah), colonizer ethnographies, and nationalist myths. A metaphor he used that stuck with me was "it feels boring to only have the keris* in our imagination." Because I am a horrible little nerd-goblin, rather than just sit with that metaphor and appreciate it for what it is, my brain decided to run with it and make an item for your TTRPG games.
*A SE Asian bladed weapon carried by characters in the aforementioned heroic literature (as well as by, you know, real people), also Anglicized as "kris" and occasionally "creese". The spelling "keris" is the one used in my edition of the Hikayat Hang Tuah, so that's the one I'm using.
The Keris-in-Your-Imagination can only be inflicted on a willing subject, through a complex ritual performed by certain members of the reigning nobility and a few of their trusted courtiers. How much of the ritual is necessary to make the magic work and how much is just slow accretions of courtly ceremony & tradition is not clear, perhaps not even to the handful of aristocrats who know the ritual well enough to perform it properly. The trappings of courtly ceremony are necessary in their own way, however; no matter how much the nobility want to frame the Keris-in-Your-Imagination as the highest of honors, it is, make no mistake of it, a curse.
The upside, which is the aspect most immediately visible to and known by those outside of the court, is what you might expect -- the creation of a keris of exceptional magical ability. The keris created by the ritual is only semi-real, being -- again as you might expect -- stored in the imagination of the warrior who was subjected to the ritual. Said warrior can, at any time, without any extra time or effort required (as a "free action" in D&D terminology), manifest the keris into reality at any point within five feet of themselves that is not already occupied by another person or object. (Usually in their hand, for efficiency's sake.) It can likewise be demanifested at will.
The form and properties of the keris are extremely flexible -- each time it is manifested, it appears as the warrior desires it. Its aesthetic properties (e.g. carvings on the hilt) are often consistent from manifestation to manifestation, being as they represent the ideals and preferences of the warrior, but its functional qualities, such as the length and sinuosity of the blade, are regularly adjusted for the needs of the hour. It is, of course, not a mundane weapon, but what enchantments are on it also vary from manifestation to manifestation. Each time the warrior manifests the keris, it has whatever enchantments and magical properties they wish it to have in order to serve their current needs.
N.B.: Some limitations should be placed on the strength of the enchantments that can be applied to the Keris-in-Your-Imagination, which will obviously vary system to system. (As an example, in D&D 3.5, you might say that the various abilities and bonuses cannot add up to greater than a +5 enhancement.) At the GM's discretion, the keris may manifest differently than the warrior intends, in response to needs of which they are not consciously aware (e.g. manifesting with a cold iron blade when confronting one of the fae, even if the warrior is not aware of the otherworldly nature of their opponent).
The downside -- and this is where the curse comes in -- is what is removed from your imagination in order to make room for the Keris. Very much by design, the space the Keris takes up in your imagination is the space in which you might otherwise be able to imagine another way to live. Once you have the Keris in Your Imagination, you are bound to a certain way of existing, as the perfect feudal subject and paragon of manly warrior virtue. If you are a fan of Asimov, it might help to characterize your new way of being in a fashioned patterned after his Three Laws of Robotics:
Thou shalt not do anything to harm the monarch who granted you the Keris-in-Your-Imagination.
Thou shalt not disobey the monarch who granted you the Keris-in-Your-Imagination, except where it would conflict with the first law.
Thou shalt not violate the warrior's code of honor (whatever that might be in your setting), except where it would conflict with the first or second laws.
There is no save to resist this compulsion or ability to struggle against these constraints. You do not see these as rules imposed upon you from outside -- you cannot imagine that other modes of behavior are possible, so you cannot fight it. Even if someone were to present to you a perfectly valid reason to turn against your monarch -- it is as if they had asked you to draw a triangle with five sides. You cannot even imagine what it would be like, much less take steps towards accomplishing it.
It should be noted that the Keris in Your Imagination is most often given to those who display martial prowess, not necessarily the most loyal servants -- those on whom the Keris is bestowed will become loyal, so their pre-existing loyalty is not a concern. However, there is a small side effect that may occur if the person inflicted with the Keris is not naturally a loyal servant. Since the Keris takes up the space within your imagination that might otherwise contain any thoughts of disloyalty, it may be the case that in situations where you would normally consider disloyalty, you involuntarily manifest the Keris into your hand. You may also have the impulse to then use the Keris -- why would you have manifested it if you did not intend to use it? This quirk is generally smiled upon by the monarch who cursed you; responding with violence to anything that would have otherwise provoked thoughts of disloyalty only leads you to more deeply fall into their service.
The Keris in Your Imagination makes you the perfect feudal warrior, but also makes it so you can never be anything else.