Seems simple: you have a powerful item that you don't want the BBEG to steal and destroy or use against you. Are there any effects you can apply to it via crafting that will help keep it in your possession or let you instantly take it back?
And if not, can you at least craft effects that cause harmful side-effects to wielders other than yourself (like how the life-drinker gives negative levels to wielders who aren't undead/constructs) or make it weaker when others use it (like how the holy avenger has a lower enchantment bonus and fewer special abilities when used by non-paladins)?
Best Answer
Yes
Here are some:
Custom Magic Items. The custom magic item rules in the DMG suggests that a DM should give a player price reductions when crafting an item with additional restrictions on usage. That seems silly, but it is what the text says. In particular, you should get a 10% reduction in price if your item requires a skill to use, and a 30% reduction in price if it requires a particular race, class, and/or alignment. Since the rules for this are in the DMG and are phrased as part of the DM's process for estimating the gp value of custom magic items, this method may well be forbidden to players in any particular game.
Sentient Magic Items. The Sentient Magic Item rules let you create a being with a Special Purpose and phenomenal cosmic power that is leveraged in the furtherance of that purpose. One possible purpose for items is "Defend the servants and interests of a specific deity" which could be you, provided you are a deity. Even without claiming the mantle of godhood, an intelligent item can be excellent protection against theft or misuse. A wielder whose alignment is not in accord with the item can suffer up to 3 save-less unremovable negative levels, as well as forcing a Will save to avoid being forced to, e.g., immediately surrender.
The -2 Cursed Sword allows a wielder who bears it for at least a week to draw it automatically whenever they wish to draw any weapon (we usually run this as the sword teleporting into the wielder's hands). As long as you draw it at least once a week, then, no one else will be able to establish such a bond with the weapon and you needn't worry about losing it to theft. You can upgrade the sword, just like any other item, with the Craft Magic Arms and Armor feat.
The locked gauntlet keeps you from dropping or being disarmed of a weapon. It's also a mundane piece of equipment, so there's no real reason not to have one or several.
Traps can keep people from messing with your stuff. Especially magic traps. For example, glyph of warding or explosive runes or symbol of pain.
The first two of these countermeasures are easily defeated via UMD and protect against usage but not theft. The last one requires a casting of limited wish before theft can be practically attempted, and that spell allows Spell Resistance. I have used the -2 cursed sword before to safe-keep expensive custom magic powers and have found it very useful, particularly if it can be used in conjuncture with sentience and a race/alignment/skill restriction.