l put something like this on the BBG site. Playing with the Ancient One Atlach-Nacha the first Mystery card i draw It says, "If an investigator would gain an Artifact, he may gain the Crux of Cykranosh Artifact instead". then "At the end of the Mythos phase, an investigator with the Crux of Cykranosh Artifact may lure the Spider-mother's brood into the open, spawn The Brood Epic Monster on his space". l have to get the Artifact Crux of Cykranosh to fetch into play and defeat The Brood Epic Monster. I do not have the Artifact so the Epic Monster is not in play yet.
I have the Old Journal Unique Asset which i passed the test then flipped over the card which said Advance the active Mystery by 1, does that mean i have beaten the Mystery?

2 hours ago, alboy said:

l put something like this on the BBG site. Playing with the Ancient One Atlach-Nacha the first Mystery card i draw It says, "If an investigator would gain an Artifact, he may gain the Crux of Cykranosh Artifact instead". then "At the end of the Mythos phase, an investigator with the Crux of Cykranosh Artifact may lure the Spider-mother's brood into the open, spawn The Brood Epic Monster on his space". l have to get the Artifact Crux of Cykranosh to fetch into play and defeat The Brood Epic Monster. I do not have the Artifact so the Epic Monster is not in play yet.
I have the Old Journal Unique Asset which i passed the test then flipped over the card which said Advance the active Mystery by 1, does that mean i have beaten the Mystery?

Unfortunately, no.

"Advance the active mystery" allows you to either place a token or clue on the active mystery, or to deal 2 damage to an Epic Monster spawned by the active mystery. In this case, The Brood Epic Monster does not spawn until the requirements are met, so this would be one of the cases where the "advance the active mystery" effect is wasted.

The problematic part of your issue is " the Epic Monster is not in play yet ".
But the answer to your " does that mean i have beaten the Mystery? " question will always be " No. " as reducing The Brood's toughness by 2 is not enough to defeat it. So your actual dilemma becomes irrelevant in the current version of your post.

13 minutes ago, KBlumhardt said:

or to deal 2 damage to an Epic Monster spawned by the active mystery.

The rules say:

Quote

If the active Mystery requires an Epic Monster to be defeated, place two Health on the card. The Epic Monster’s toughness is reduced by one for each Health on the active Mystery.

1)Reducing monster's toughness is very different from making monster lose health. While the monster needs to be around to lose Health, it's toughness can be modified before it's spawned. An example of this situation are Cultists of Syzygy.
2) The rules don't state that the monster has to be spawned. You might have noticed that the health tokens are put on the Mystery card, not on the Monster token.

1 hour ago, tsuma534 said:

The rules say:

1)Reducing monster's toughness is very different from making monster lose health. While the monster needs to be around to lose Health, it's toughness can be modified before it's spawned. An example of this situation are Cultists of Syzygy.
2) The rules don't state that the monster has to be spawned. You might have noticed that the health tokens are put on the Mystery card, not on the Monster token.

Huh. Good catch. I guess we've always done that wrong (although I'm not sure it ever really came up).

1 hour ago, tsuma534 said:

The rules say:

1)Reducing monster's toughness is very different from making monster lose health. While the monster needs to be around to lose Health, it's toughness can be modified before it's spawned. An example of this situation are Cultists of Syzygy.
2) The rules don't state that the monster has to be spawned. You might have noticed that the health tokens are put on the Mystery card, not on the Monster token.

Huh. Good catch. I guess we've always done that wrong (although I'm not sure it ever really came up).

Gotta say, that's awesome. Usually Epic Monsters spawn right away, but great errata catch.

11 hours ago, tsuma534 said:

The rules say:

1)Reducing monster's toughness is very different from making monster lose health. While the monster needs to be around to lose Health, it's toughness can be modified before it's spawned. An example of this situation are Cultists of Syzygy.
2) The rules don't state that the monster has to be spawned. You might have noticed that the health tokens are put on the Mystery card, not on the Monster token.

Thanks for the great explanation, i fully understand now, as they say if don't ask you will never know.