I am intending to have a murder mystery for my next game, set in a high fantasy setting in 13th Age. The setting we are playing in is very close to Eberron in tone, where magic is prevalent and powers industry and economy.
The problem I am having now is coming up with a plausible murder mystery scenario in a setting where you have disguise self (hence any alibi cannot be trusted), illusions/domination (hence witnesses are not reliable), closed environment not really being closed (dimension door, teleport, pass wall etc.) and monster summons (hence not needing a murder weapon).
While 13th Age doesn't have many of those spells available, it is possible that NPCs have it, as implied by the setting. It's the sense of "magic can do anything hence there is an infinity number of ways the murderer could have killed the victim!"
How can I
design a mystery that can be done via magical means, but any classes, not only casters, can solve it?
design a mystery such that the players can rule out magical means without requiring an encyclopaedic knowledge of magic ("Pass wall couldn't be used here because it is the wall is lined with lead" kind of thing). I could have magical defenses, but I don't want too much contrivance ("Oh, it's the THIRD murder scene where it is warded against magic. How convenient!")
explain why there still could be clues lying around despite it that there is magic (say, what sort of clue should a dimension door or magical portal leaves behind?)
show the players that the murderer is using mundane means so that they won't suspect a caster did it -and- why the murderer could use mundane means when they have magic at his/her disposal.