Stuff might be easier to move across planes than creatures, so if fiendish essences are being infused into some items, they might get shipped over and some cultists could perform a ritual, and viola, your fiend is loose in the world. If you use a teleporting spell that lets you take one willing creature with you, and you have a sentient weapon, the weapon usually doesn't count as your +1.
The downside is that the item might get lost or some caster with planar binding might get it instead of your cultists, so I can't see mid to high level fiends doing this themselves; this is the kind of thing you inflict on imps, quasits, maw demons, spined devils, and maybe the odd bearded devil, hell hound, or nightmare. It might be an award to some noncasting cultist to get their own "servant."