To find Vorres the player must have spoken to the "Huntmaster" in Phon Coast and defeated Thalassinon, which will trigger the start of the Rare Game quest.
Vorres then appears within the Necrohol of Nabudis in the Hall of the Ivory Covenant area. To get it to appear, the player must find a Dark Elemental, provoke it, and lure it to the door that leads to the Cloister of Reason (where the optional boss Fury is located), right in the middle of the map. The Dark Elemental is found just north of the door, and when it is in the correct location (excited), Vorres appears in the room south of the door.
Like all Trophy Game, once it is killed, it will not reappear, so be sure to steal the Soul Powder it has before killing it, an item required for making the Tournesol. Multiple Soul Powders can be obtained by stealing the loot without killing Vorres, then retreating two screens away and returning to lure the Dark Elemental, spawning Vorres once more.