I was with a pty, and we were chaining imps. Not once during any of our fights did a imp call for help. This was just a purely random drop. We had no thief, nor did anyone have /thf sub, so I'm not sure if TH does or does not affect this drop rate. I would imagine it does.
I tried farming this for my bonecraft.. but I realised that horn won't drop unless it uses it during battle, just like the description says. I'm wondering if it's a 100% drop rate for the horn if the imp "calls for help" during battle, making Treasure Hunter unecessary?