The Gaslighter archetype for Mesmerist has an ability called Horrid Mask:
Horrid Mask (Su): At 3rd level, as a standard action, a gaslighter can implant a maddening phantasm in the mind of the target of his hypnotic stare, causing the target to perceive its own reflection as corrupted in some disturbing way. This is a mind-affecting fear effect, and there is no saving throw. The target is simultaneously captivated and horrified by the sight of its own reflection, and in any round in which the target can see itself in a mirror within 30 feet, the gaslighter can trigger his painful stare against the target without dealing damage. This is a gaze attack, and the creature can avert its eyes to attempt to avoid it. Any creature holding a visible mirror that attempts to demoralize the target can choose to add the gaslighter’s Charisma modifier to its Intimidate check instead of its own. Horrid mask lasts until that particular hypnotic stare ends (often when the target moves away from a mirror). The gaslighter can use this ability a number of times per day equal to 3 + his Charisma modifier.
Which damage is dealt for this? Your personal damage, or the ally damage?
What happens if you apply Manifold Stare to this ability? Do you get to start your turn at 9th level applying 3 lots of your stare damage and debuffs to this target?
On that note, how much damage does the mesmerist actually DO with his painful stare? At 6th level, if my mesmerist of strength ten hits with a dagger for 1d4 damage, does triggering my painful stare deal 1d4+2d6+3? (The word increase is the trigger here, in the painful stare description.)
In part, yes. You don't need to deal damage to trigger the painful stare, I assume the stare does it's damage as normal, since that's the Painful Stare's only actual function. So how much damage does the stare do? As a sixth level Mesmerist, is it dealing 3 damage, or 2d6+3? Or is it just 2d6? Hero Lab seems to be under the impression that it's 2d6+3 when I cause it to trigger instead of an ally, but is that from Horrid Mask as well?
I would say it's 2d6+3. The Mesmerist himself is triggering it, so gets his personal bonus damage.
As far as Manifold Stare, remember that it's a gaze attack. That means you get one free application on their turn (if they don't look away), and you can choose to blow a standard action to make an additional gaze attack on your turn, which I'd allow with a single instance of Manifold Stare. There's no way to apply a gaze attack three times in one round to a single target.