Frightful Presence Line of sight or Line of effect?

[Guy Ladouceur]

When a dragon for example utilizes their frightful presence which rule applies line of sight, line of effect or just overall range to affect those within the range of the ability, say 300 ft?
If line of sight is not needed and the creatures affected cannot see the dragon (total darkness) do they actually know which direction the dragon is, in relation to those affected?

[*Thelith]

Frightful presence says:
"Opponents within range who witness the action..."

Unless they can see it happen... They are unaffected.

[Tim Emrick]

The OP also asked whether a character who can't see the dragon can tell where the dragon is if they can only see the creatures affected by its frightful presence. I'd say that would be a good time for a Perception check. A dragon using frightful presence is unlikely to be stealthy at that moment.

[*Thelith]

If you can't see him to be affected by a frightful presence, then he is essentially invisible. You could possibly determine the direction, and obviously you may hear him charging via a perception check but he has total concealment via darkness.