The NA instruction booklet does rather imply that it is only possible to attack a monster rather than forbid in words to attack another hero:
p. 13: "As a Hero you may attack any monster that you are adjacent to."
Vice versa:
p. 22: "A monster may attack any Hero that it is adjacent to."
For spells, the cards state who the spell can be cast on.
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Let me know if I succefully did the Daedalus here.
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I rewrote the rules in such a way that everything - attacks, spells, skills - can be made/cast on/against "any one figure" and leave the choice to the heroes and me what to do with our figures. But trust me, as a hero group you do not wanna backstab your way through my dungeon or put a healing spell on a Fimir just for fun.
ghan wrote an awesome questpack,
Champion of the Empire, that lets the heroes choose the path of light or of darkness. They then are opponents if different choices are made, sometimes openly, sometimes secretly.