I'm new here so hi to all

1st question:

setup: we have Arkham Horror and Kingsport Horror , all cards from Dunwich Horror except Mythos Cards and game board from Dunwich, all small expansions except Curse of the Black Pharaoh.

Yig is the Ancient One, and Lurker at the treshold is herald, bast is our guardian but nevermind her.

Is it legal to do as follows?

Get "Call Ancient One" spell from Black Goat of the Woods expansion [by using Livre d'Ivon for example], then collect 11 monster trophies and cast "Call Ancient One" by taking pact from Lurker, so sanity cost is reduced to 0 and spell check is instantly passed. So poor Yig is reduced to protoplasmic slime even without fighting him.

2nd questions:

Can I convert all my sanity to power by using soul pact from Lurker at the Treshold, go to Arkham Asylum and draw a madness card and regain sanity?

Your first question wouldn't work. You can only discard the number of trophies that the sanity is set to. If you cast the spell using zero (setting X) sanity, then when you discard X trophies, since you've set X to zero, than zero trophies is all you can discard. Which means X doom tokens removed becomes zero doom tokens removed. Note that this is because the Lurker reduces the sanity cost to zero, it does not bypass the sanity cost. So using the Lurker for this spell, you have no choice but for X to be zero.

As for your second question, I believe you are allowed to do this.

Yeah, the Lurker herald is a bit too exploitable. Hopefully the upcoming FAQ will clarify that herald for the better (read: worse).

Thank you, DoomTurtle. I had forgotten about that logic and was about to say that CAO+Lurker was allowed. I forgot that 0 sanity cost means exactly that.

Now, Qwicol, pick up the Ancient Language skill and Crystal of the Elder Things, and that's a different story

But note that, last I knew, you can't cast a spell with a higher sanity cost than your current sanity, even if you can prevent or bypass some or all of the loss. I know that I suggested this as a house-rule long ago, but I thought an official answer to someone's recent email clarified that. I can't remember if it's in the FAQ but I'm not allowed to say whether it is or not or what it says anyway, so it's anyone's guess. All in all though, it is a good rule to use, if for no reason than to make CAO (and Shroud of Shadows) more reasonable.

If you don't use this rule, then there's nothing stopping you from setting the sanity cost of Shroud to 1000000 and preventing it all with the Crystal. That ain't right, I tell ya.

Well, Harvey Walters with "Changed" card from Kingsport Horror still can wreak havoc upon Ancient Ones with CAO and any Dark Pact, but he is like 70% of Call of Cthulhu RPG investigators merged together to save the mankind, so its ok

Spectral Razor is also unbelievable powerfull spell, imagine the immesurable cosmic energies you may have between your fingers when casting it with Blessing and Library Use. Unfortunately you can't bypass magical immunity

Well, thanks for answering my questions.

Tibs said:

Yep, the Visions special card could be exploited the same way without this rule. Discarding an X sanity cost spell would hypothetically yield infinite clue tokens if you don't set some rational limit on the sanity cost.

Actually, with Visions, these cards would give 1 clue token because their sanity cost is 0 unless you're attempting to cast them.

You're right. My bad.The value of X is set only when you pay the sanity cost of the spell upon casting it. You can't arbitrarily determine its value otherwise.