on some feared units it does nothing. you can still attack the unit despite being feared. however the big weird interaction is unspeakable and fear. if a yeti howler fears you with horrific aura, you cant be attacked for the turn. however if you play something with unspeakable on the following turn it reapplies the fear plus the fear of the unspeakable. extending the duration for another turn and so on for the next turn if you play another unspeakable unless cleansed.
I'll confirm this and call more attention to it, since I keep accidentally abusing it in my Yeti BG. It gives no indication that the champion is still afraid of the Horrific Aura unit, but the duration is definitely refreshed. Not sure if it ever refreshes other sources of Fear.
