Headbang on the top floor of the slayer tower
Warning! Potential danger for hardcore ironman!
1. Collect a Seercull, combat bracelet and a helm of Neitizot. Seercull is a rare drop from the Dagannoth Supreme boss, a combat bracelet can be bought from the pures shop at the ::shops at home and a helm of neitiznot can be bought from the armor shop at the same place. Ironman can not buy Helm of Neitiznot, they can receive it as a rare drop from Yaks, which can be found under the training teleports.
2. Training teleports > slayer tower
3. Cross the tower and go to the top (2nd) floor.
4. Equip above items and perform the 'Headbang' emote, a level 141 double agent will spawn, kill it to finish the clue!