Grog said:
Inscribing a symbol on the inside of a chest you want to protect doesn't match up to any definition of "offensive" that I'm aware of. The spell isn't being used aggressively or as part of an attack, so that usage would be fine.
I totally agree. I was asking, however, for a usage that you thought might go straight up to the line but not cross it. Let me give some examples of things that I'm curious as to how you'd rule:
-In an effort to assassinate someone, I inscribe the symbol on the door of his closet. Offensive or not?
-I inscribe the symbol on the front door of my house, and then invite my victim over for cookies. Offensive or not?
-I inscribe the symbol on the front door of my house, and then warn the victim not to come over, knowing that he'll take my villainous warning as an invitation to come over and kill me. Offensive or not?
-I inscribe the symbol on the front door of my house, and then warn the victim not to come over, knowing that he'll take my warning at face value. Offensive or not?
As far as I can tell, the line between offensive and non-offensive use of the spell is pretty blurry. As a DM, I'd rule very conservatively, using the spell's examples as the type of offensive action that doesn't work. I like it when stuff works, though, so that may be my bias

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Daniel