This sounds like an offensive Trap. Boulder Trap I used offensively as well (The only time I ever used it defensive is with special usage as I didn't want to lose anything) so fitting for it to be similar to the Boulder Trap at appearance if I read that correctly.
I wouldn't dare put it in my own dungeon for the sake of my own belongings, don't care about my enemy though...
Me: Hey man, how's it going!
Enemy: Uhh... fine?
Me: Cool! I got you a present, let's be freinds.
Enemy: Ah sweet! Thanks dude! *Places Reaper Trap near Dungeon Heart*
Me: Ohh, can I bring visitors to your dungeon?
Enemy: Sure sure!
Me: *Sends to Dungeon Heart*
: *Sets off Reaper Trap, killed*
Me: Heh heh heh... *Watches Enemy die and bleed*
That would be sweet... I need to try that sometime, lolz.