Harrigan is going to take out the Haunter in the Dark from the Mystery Card of the same name.

He has only one Magical Possession The Shining Trapezhedron.

The Shattered Magic Mythos Card states he has to discard half his Magical Possessions. Rounding up (reference guide). It isn't double sided, so it's out of the game.

Haunter in the Dark Mystery Card.

The Errata states that unless you have the Shining Trapezhedron you choose and encounter as if the Epic Monster isn't on his space (you can't see it I presume).

Therefore, how do I kill it if I can't see it?

Does this mean that the only way to complete this Mystery is via other means, such as reducing it's health to zero by some other method, but not being on it's space. This seems a tad on the rough side and a little pointless for a Mystery that needs solving..

Am I missing something?

Posted on BGG also.

Ok, have my answer to this. Thanks.

But your note gonna tell us the secret answer?

Possible spoiler.

Well, ok. You can just recyle the Shining Trapezhedron as it's a specific Artifact. But the way the errata is worded, you don't even have to go to Arkham to confront it.

When this card enters play spawn the Haunter of the Dark Epic Monster on Arkham. An investigator on Arkham chooses an encounter as if the Epic Monster is not on his space unless he has the Shining Trapzohedron Artifact.

Tokyo lets you damage (not discard/ move etc etc you can't do that with epic monsters) monsters from afar and certain tasks and expedition cards allow you to advance the current Mystery (in this case reducing it's toughness by 2).

And unless it's specifically states "A Non-Epic Monster" it's refering to any monster on the board.

It took me a reasonable 3 turns, with 4 investigators, without camping on Tokyo nor raiding expedition encounters. Seems almost too easy for a Mystery.