
A solid pick for Diana Stanley if you're building clue focused. In my deck I went from enchanted blade to this. I'm trading off a bit of damage but gaining flexibility. We're also freeing up an arcane slot.
This is great for killing 1 hp enemies like cultist and rats etc getting your base fight upto 5. When you don't have to worry about enemies because your teams fighter has things handled you get a +2 on your spells which is really helpful if you haven't maxed the number of cards under you. The core argument of "boosting something you're already good at" is exactly where this is so good. There will be some turns diana isn't as good at what she's supposed to be good at (willpower). This can also cover your back when your weakness cleans out your stored up cards. Lastly it can be dual wielded for a bigger one time boost or two uses. This should either be before you get your signature weapon or in conjunction with Bandolier.
Bandolier (2) is another place you could make solid use of packing spirit athame alongside your signature & enchanted blade. This lets you get a bigger fight boost than the base enchanted blade and allows saving the charges for when you really need it. The bonus from Bandolier(2) is a welcome one for increasing your base heads. Possibly more of a play on a flex build.
I like sixth sense on diana so I don't have to worry about charges once setup and I also use Alchemical Transmutation to make me money to afford the plethora of expensive assets I run. The spirit athame works great with both of these.