Let's look at this card line by line.

1. It's unique. You can only have 1 in play.
2. It cost 3 which is a decent amount of resources, especially for mystics.
3. It's both an item & a relic. It's cursed.
4. It costs 0xp and can be in your starter deck.
5. When commited to a skill, it adds , which isn't as great as for mystics.
6. It seals a , , , or removing it from the chaos for the entire team. This is very powerful.
7. It only returns to your hand when YOU reveal the eldersign.
8. It takes up one of two hand slots, which are seldom used by mythics.

First analysis is that it's very strong.

Math suggests that with 16 chaos tokens in the chaos bag (15 after playing) while performing 2-3 skills tests a round it'll last an average of 3-4 rounds before returning to your hand. This is still largely chance based. This equates to 1 resource per round.

Pairing with cards like Time Warp or Seal of the Seventh Sign can add to this cards duration.

In multiplayer, playing this as a final action is a great benefit to the whole team, especially if you go first.

Can be searched for with Backpack or Dr. Elli Horowitz

Able to cast after Emergency Cache.

Now lets compare to Protective Incantation who is very simular.

THE CHTHONIAN STONE

• Pro: Uses hand slot.
• Pro: Overall cheaper per round than Protective Incantation, but also luck based.
• Pro: Starting card.
• Con: higher initial cost.
• Con: Unique item. Limited to 1.
• Con: Commits a .

PROTECTIVE INCANTATION

• Pro: Able to have 2 in play.
• Pro: low initial cost.
• Pro: Able to remove 1 chaos token for entire scenario.
• Pro: commits
• Con: Upkeep cost
• Con: Uses arcane slot
• Con: Costs xp.

Final Anslysis: I Think The Chthonian Stone is a great card, but probably only as a one-of as the second copy is dead in your hand and only commits a . Most scenarios have one token that is worse than others & this can seal it up. It's extremely powerful in multiplayer & gets stronger with more members. You normally get 3+ rounds with it in play, with Time Warp you can double it's duration. Cheaper per round than than Protective Incantation and doesn't use a valuable arcane slot for mystics.

You could run 1x The Chthonian Stone and 2x [Protective Incantation] (/card/04031) and seal up to 3 chaos tokens, but that gets expensive really quick & there is no easy way to do this keep up the resource dedication.

I especially like this card for Jim Culver and Norman Withers decks.

It's not immediately clear how this card interacts with Live and Learn. Does Drawing Thin apply to both attempts or just the first one? There was some debate about this, but the final ruling came from Matt Newman himself:

Greetings,

This is a bit of a tricky interaction, so I apologize for any confusion here. I agree it’s a bit ambiguous. I think the ruling that makes the most sense here is the following:

As a general rule, when you use Live and Learn to attempt a test a second time, all effects with a duration that expire at the end of the first attempt will have expired by the time the second begins. This includes effects used during the first attempt that say “until the end of the skill test…”, “…for this skill test,” or the bonuses from committed cards, which are all discarded at the end of the first attempt. Effects that are inherent to the test itself (the test’s parameters, what happens if you succeed/fail, that sort of thing) all remain the same, even if they have a duration of “for this test.” So, for example, if an effect said “play during a skill test. until the end of the skill test, increase the test’s difficulty by 2,” that would expire at the end of the first attempt, whereas if the test itself said “Fight. Increase the difficulty of this test by 2,” that increase in difficulty would exist in both the first and second attempts.

Now for the tricky part: Which is Drawing Thin? Is it an effect that initiates during a skill test with a duration of that expires at the end of the skill test? Or is it an effect which alters the inherent nature of the skill test itself, such that it would affect both attempts? Since Drawing Thin does not explicitly say any variation of “until the end of the skill test” or “for this skill test,” and since its triggering condition is a “when” reaction to the skill test initiating and not something you use during the first attempt, my ruling is that Drawing Thin is changing the skill test’s inherent difficulty to be 2 higher—altering the nature of the test itself. Therefore if you use Drawing Thin when the skill test initiates, and then use Live and Learn to attempt that test a second time, the increased difficulty would carry over to the second attempt.

Again, apologies for the trickiness/ambiguousness. Hopefully this clears up this interaction, as well as clearing up how Live and Learn works in general. Thank you for bringing this to my attention; I’ll be sure to add it in the next edition of the FAQ as well.

Cheers!

Did they forget to make this fast? It might have been worth considering if it was. As it stands, if this card isn't in your opening hand then most of the time you are effectively paying three resources to get +2 in a skill test, the equivalent of Unexpected Courage. Comparing it to the other neutral skill cards like Perception or Overpower is even more unfavorable because those allow you to draw a card if you succeed and are completely free. There are a handful of situations where the Ace of Rods will give you +4 in a test, but even then it is still too pricey to seriously consider.

One positive is that because it gets removed after use, putting two in your deck doesn't result in a dead draw. Preston Fairmont can use it as a means of flexing on other investigators to show how little he cares for efficiency.

The other review is underselling this card. At least on standard difficulty I think this is one of the top 5 best level 0 Seeker cards ever printed and a card that I shove into most of the decks that have full access to level 0 seeker cards. Comparing this to No Stone Unturned this card only digs 3 deep instead of 6 but doesn't require an action or 2 resources to use. This card is very easy to just throw onto any test that you're all but guaranteed to pass anyway (such as investigating a low shroud location) and then use it to have Eureka be replaced by whatever's the best card in the top 3 of your deck. The icons it has are fantastic, it's basically the only 3 skills you're likely to ever use so it can be committed to almost every test you take. On top of that, there's a lot of tests you might take where you're +3 above the test for example and passing it is important, but you don't want to spend any of your valuable cards to boost yourself up to +4. Eureka is great in those situations because you can insure that you are protected while also having the card cycle into something you need.

The card is just super versatile and is always useful, It's not necessarily a card that'll win you the game but it'll help you find those cards and is never a bad draw.

I've just recently started a 2 gator playthrough of Cirle Undone with Jim. I'll have a deck list to come soon but at the moment it has gone well using the sealing cards as well as Defiance to mitigate the bag in Jim's favor. Most every test so far as long as I can at least match the test value has gone my way. I feel like sealing mechanic may have jumped up Jim's power level by quite a bit.