Violet eyes.
The man's brows drew into a knot as he thought of the various monster out there, yet couldn't seem to recall one with violet eyes. Besides, according to his third son, the figure was too human-like despite having two horns, scally skin, and wretched wings. And not only 'human', Tristan had associated that thing with 'intelligence'.
Although there were some monsters possessing intelligence like goblins and trolls, especially so when they reached a certain level, it was still nothing worth mentioning. But, if Tristan believed that this so-called demon was as discerning as humans, then it was highly possible that what Tristan had seen was indeed a demon.