Plot point for a Planescape mystery

Hmm, I'll give it a try.

The cornugon makes a deal with someone he 'trusts' to some extent and has known for a while. Perhaps it's an evil-aligned mortal he's worked with in the past. ("He's a mortal, he'll be easy to peel...") A certain percentage of the deva's Blood War weapons are impressively high-end, intended for baatezu officers and the like.

This 'mortal' offers to replace some of the really nice weapons with convincing but cheaper knock-off imitations (i.e., replace the Unholy Sword of Doom +5 with a regular +2 sword). Such as, for example, the weapons of two baatezu higher up in the cornugon's chain in command, who dislike him and/or have blocked his efforts to be promoted in the past. Then the mortal will resell the real weapons and they can split the profits.

Unknown to the cornugon, the 'mortal' is either a yugoloth or the mortal agent of one. Each of the weapons he 'provides' has a well-hidden magical effect on it: something along the lines of inescapable detection/location. Not only is the Unholy Sword of Doom less effective than its owner thought it would be, it's been allowing the 'loths to trivially scry on its owner for some time now! While the baatezu were drawing up battle plans, for example!

The fact that the 'loths can much more easily pinpoint the locations of important baatezu officers really makes it easier to carry off targeted assassinations. Rather than use up this advantage in one large battle, the 'loths intend to enjoy this advantage for as long as possible before the baatezu (or the PCs...) figure out the trick.