These are… not precisely free-willed spirits or supernatural entities. Instead, they are a shell into which a spirit can be poured into – preferably, voluntarily. Not primarily because of any ethical concerns, alas: it’s more like you probably shouldn’t stuff something into an independent weapons platform unless it doesn’t mind being stuffed in there anyway.
In their ‘natural’ form, Fairy Swordfish look like small, winged, bomber jacket-wearing female humanoids with a narwhal’s horn on their heads and a grenade in one hand. The species has what appears to be an almost religious aversion to hand-to-hand combat: they will happily stay out of range and rain down grenades at you from the air… unless you’re an ally of the British Crown, which is the entity that ‘owns’ the Fairy Swordfish shells. In that case, they’ll just blow up your enemies, instead. The spirits hired to imbue Fairy Swordfish are typically not evil creatures, so they don’t actively attack innocent people, but British Esoteric Intelligence is well aware that when you give flying spirits hand grenades, every so often the wrong thing might blow up*.
Nonetheless, they’re invaluable when it comes to cleaning out nests of apocalyptic cultists, assuming that the cultists were considerate enough to set up shop in truly desolate areas. Which happens surprisingly often. It’s like none of those people ever need to go to the supermarket…
*How does BEI cover up the existence of Fairy Swordfish? That’s an interesting question. Do you have the necessary security clearances to allow you to hear the answer?