This looks like it will be the one of the least impactful weakness in the entire game. Let's assume a NotZ chaos bag on Standard for The Gathering (no Blurse tokens). Assuming there is one Ghoul at your location, the probabilities of passing a test at 0 / 1 / 2 / 3 / 4 above difficulty are 25 % / 63 % / 81 % / 88 % / 94 % respectively without the Elder Sign, this changes to 20 % / 60 % / 80 % / 87 % / 93 % (probabilities calculated with www.arkhambagcalculator.net.) The change is -5 % / -5 % / -1 % / -1 % / -1 %. This is almost negligible.

Let's take a harsher bag into account (Devourer Below on Hard), the probabilities are 22 % / 33 % / 44 % / 67 % / 78 % and change to 18 % / 29 % / 41 % / 65 % / 76 %, a change of -4 % / -4 % / -3 % / -2 % / -2 % so even here, the impact isn't enormous.

So if you take 20 skill tests in the time it is active, you should at worst expect to lose one of your skill tests due to this card on which you would have succeeded otherwise. I guess it might be annoying when you draw it early on an Agenda with a high doom threshold (for example in Where Doom Awaits).

On the surface, this looks like a 50% more expensive Flashlight. Let me draw your attention to the three major differences between them:

Cryptographic Cypher lets you choose to, instead, spend a use on a investigation instead of a one, at the cost of making the shroud 1 higher.

Cryptographic Cypher uses secrets, not supplies.

Cryptographic Cypher exhausts after use, while Flashlight does not.

The first point is important for people with high intellect that would normally not take Flashlight, which is most Seekers. Getting 3 additional investigates that also potentially dodge attack of opportunity definitely has value, especially in solo where clues are distributed less tall and more wide (where Ursula Downs excels). It should be noted that Trish Scarborough can turn that into 3 investigates AND evades. Of course, this bares similarity to Fingerprint Kit, which is likely better compression at higher counts at +1 advantage instead of -1. But if you're able to consistently cycle through skill cards (especially Deduction) and commit high enough, the skill value shouldn't be an issue, and Seekers are the best at doing that to begin with (hello Practice Makes Perfect and Mr. "Rook").

The second point means you wont be able to reload this the same way as Fingerprint Kit or Flashlight, say by using Emergency Cache. You have to use Eldritch Sophist, or Truth from Fiction, or Astounding Revelation.

The third point is that this exhausts after use, meaning that you can't chain in one turn (unless you're "Ashcan" Pete). You'll have to spread its uses over several turns.

The odd thing about the card is that it still lets you use it functionally as a slightly slower Flashlight, which I'm not fully sure why someone who took it for the first function would ever want to use the second, or vice versa why anyone who would take it for the second wouldn't just take Flashlight instead (which these days includes lots of Survivors who now run Old Keyring anyway). I think, overall, this is mostly a true solo card, intended to conserve actions (net 2, though keep the costs in mind) and have that little bit of flexibility that might matter on a particularly difficult location that you only have yourself to rely on to clear. Again, Trish Scarborough might like both functions of the card- the former for dodging AoOs, and the latter for helping enable a "succeed by" test that Rogues often like to perform to refresh themselves.

This is an interesting weakness. It punishes you for keeping it out of play and punishes you slightly less for keeping it IN play.

I assume the goal here is to force Dexter to waste his special ability to get it out of his hand and then use his special ability on another asset to discard it from play?

That's a great way to attack his play style. I really like this design.

This is a very good card and only gets better the higher one's willpower stat. While it lacks attack bonuses, damage bonuses and further, requires exhausting to use, its best feature is that it uses a hand slot, instead of an arcane slot. While Wither has a similar function, it does compete with other spells for precious slots.

It is relatively inexpensive and ideal for fighting odd-number health enemies, when an early game Shrivelling or Azure Flame charge would either be overkill, or underkill. Saving a charge to finish off 1/3/5 health enemies will also extend the life of those spells.

It also allows evading, with one's willpower stat, ad infinitum. Great for mystics whose hands aren't already occupied by Trumpets, Statues, Stones or Flashlights. And as if all that wasn't enough, unlike the eldritch equivalents, this mundane weapon has no downsides for drawing special tokens. 5 will investigators like Dexter, Agnes and Akachi, often with further passive bonuses, will get a lot of mileage from this card as a backup weapon/evasion tool. And it doesn't use an arcane slot!

If you didn't already have Leo De Luca, its only 1xp more to take Charisma and 2x Leo (1) over 2x Haste.

Leo costs 2 more resources but also more than pays off that difference by generating more bonus actions reliably as well as being a 2/2 soak. And it's not like anyone who has access to rogue 2 will have problems with paying 5 upfront.

Baring edge cases like having 3 other allies already, I don't think there's any investigators without Leo who would want to take this. While Dexter Drake certainly likes having more spells, he also has better things to do with his arcane slots.