I have a question regarding the wording on the Treacherous Magic Mythos Card from the base game. It states, "Each investigator may discard any number of Spells, then he loses 1 Sanity for each Spell he has. Each investigator that lost Sanity from this effect also gains a Madness Condition." During a 3-player game, 1 investigator had 3 spells, & each of the other two had 1 spell. Does that mean all 3 lose Sanity according to the number of Spells along with getting Madness? That's a really debilitating card.

Each investigator runs through the following set of steps independently of one another:

Step 1: discard any/all/none of the spells that they possess

Step 2: lose one sanity for each spell that they did not discard

Step 3: if they lose one or more sanity from the prior step (*), then gain a madness condition.

In your case it sounds as if no investigator opted to discard any spells, in which case the investigator with 3 spells would lose 3 sanity while the other two would each lose one spell. Then, since each investigator lost >0 sanity, each would draw a madness condition. And yes, depending on the timing, this can be a devastating card.

(*) It's possible that an investigator may have some spells, but not lose sanity if they have a way to cancel it (e.g. a Whisky asset to discard.

OK. That clarifies the explanation, & is not as bad as I thought. You discard Spell cards to prevent/reduce Sanity loss & the Madness. Perhaps if the card had read, "Lose Sanity from Spells you have left ." Thanks.

Using "left" implies that you're required to discard at least 1 if possible; the actual wording is sounder because it allows the two parts of the Mythos to be actually independant.