The Potion of Reveal reveals objects of deception, illusion, or invisibility. Connor collected them during his quest allowing him to find hidden secrets and see invisible enemies.
With the use of these potions Connor was able to find a hidden chest inside of the Wizard's House, see the Apple of Nakedness in a tree near Sarah's House, see that the Strange Door in Dimension of Death Jail was an illusion, and he was also able to see the Snow Mane in the Frozen Reaches.
He discovered four of these potions total during his quest and had to choose carefully how he used them. One was found inside of the Wizard's House, one inside the Hall of Immortality, one on top of the Northwest Tower, and a final one inside of the Fire Dwarf's mines.