Interesting and powerful card, that is released in the same box as the upgrade of Sign Magick, for the same amount of Experience, and the same resource cost.

Nevertheless, I will prefer Sign Magick to The Hierophant • V for the Fast Aspect, to keep the Tarot slot free for Four of Cups, and most importantly for a free trigger of another spell: You killed an enemy (or not btw), get a free investigation (without AoO); you didn't kill the enemy, get an extra attack or an evade to dodge the damages.

Replacing Four of Cups for The Hierophant • V to have an extra slot to play Holy Rosary is the same as paying 2 XP and 2 resources for 2 Sanity. (Mr. Pawterson is looking at you). Crystal Pendulum and St. Hubert's Key are barely better in that slot.

You're therefore more likely to use this card to improve the size of your Arcane pool.

Is it the best card in doing so?

I don't think so. Familiar Spirit and Sign Magick (0) are lvl 0 cards, and Sign Magick (3) offers insane options for the same amount of XP and using a slot that is less contested in Mystics than other classes.

For a more detailed review of both cards and more info on why I believe that Sign Magick is better, see the review of Sign Magick.

These new Tatort cards seem to be pretty strong for sure if they are released like currently perceived in the pre-release teaser over on fantasyflightgames.com. Honestly, this card adds a whole new dimension to the Relic Hunter. Relic Hunter does not take up deck space, so adding this card and Relic Hunter will give a Mystic a total of 5 slots usable as arcane- or accessory slot as the mystic needs it. With this the Mystics who want to wield the Enchanted Blade like Diana Stanley or Akachi Onyele can do so, you can have 3 active spells at the same time, 1 fight, one clue, one evade. The possibilities are huge and up to this point you had to give up hand slots for cards like Sign Magick to gain just 1 arcane slot, now you can more than double these slots, if required, by combining a permant and a tarot card. It is a 12 XP expansion to a Mystic though, but XP well spent in the right situation I assume.