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.