Well I did a search for Hellfire Guards and nothing came up so here it is
1) I Used a succubus, have it set to Defensive and have Soul Link turned on
2) Ive tried this everywhere, and only works for places your Neutral with check that out in the reputation tab
3) Use Hellfire in sight of any friendly guards
4) Have pvp turned on (/pvp)
You should spawn multiple guards hf attention whoring
(Im presuming the reason for this bug is that your indirectly doing damage to your pet via soul link and hellfire, havent thought of any interesting applications for this, if you do please post some!)
Edit: Added a pic
Edit#2: Added requested UI pic