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Well, there's the "Use what you can't see" which is the main clue, then also the elements "Fire, Air, Water, Earth" and the states "Gas, Liquid, Solid". Everything else seems to be irrelevant. Also, maybe the "use what you can't see" clue literally means use the blind cipher? Anyway.
Well the elements are related to things like alchemy and witchcraft... Sadly we're not gonna get to talk to ghosts so something related to alchemy seems more likely...?
hnnnnggg  |
It's been great reading through all this info. Though the solution is much simpler than you're thinking. You have everything you need and you're looking in the right direction. Try focusing on what's missing.