It'll take me a while to reproduce this, but here's what I'm fairly sure happened:

I'd unlocked South Pacific and was wandering around there trying to amass the 30 investigation points to raise Ryleh.  One of my characters had 16 trophies, so I tried to buy one of the Cthulhu relics from the ship shop.  Instead of getting one of the relics, I received 6 investigation points; and I'm fairly sure lost all 15 trophies that the relic would have cost.  Has anyone else experienced this?  Am I hallucinating and it really only cost me 4 trophies (and the game won't let you buy a relic until Ryleh rises)?

One of the relic rewards is investigation points, although I don't recall how many.  It does seem a bit absurd to get that particular reward from the shop.

 I see the problem.

One can only acquire relics after R'lyeh has risen.  Prior to rising, if you get a relic (either as a trophy purchase or an adventure reward), you get investigation points toward locating R'lyeh.  This rule doesn't seem to be anywhere present in the 5 or 6 page help section for Cthulhu.

Further, you simply should NOT be able to spend the 15 trophies on a relic (which turns into 6 investigation points) prior to R'yleh appearing.  That needs some fixing.

EDIT:  Ok, now I think I understand what's going on.  The Relic symbol does not function like the Elder Sign symbol, in that you are guaranteed it (either as a purchase or reward).  There is a relic deck from which cards are drawn, and amongst the cards are the 3 relics, some -1 dooms, some investigation points, and some seriously awesome items (Cabala of Saboth is just some kind of amazing and I have a sinking feeling it's not going to work on Cthulhu.)

fireduck said:

I see no reason why it wouldn't.  You don't even need one of the amulets to empower it.

When I defeated Cthulhu, though, it was with a shotgun.  And the Sword of Glory, and some glyph-holding spells, but the final blow was delivered with a shotgun, and there's just something totally awesome about that.