The Rock Troll can be found in the Cyclops Valley, otherwise known as Mount Dartin and on the last level of Mountain Dungeon. When defeated the Rock Troll will sometimes bleed poison and cover three spaces on the screen. Walking into the poison will deal the player 1, 2 or 3 attack every time turn until the yellow poison bar at the bottom of the screen runs out.
The poison can be used against other Rock Trolls or Cyclops in the valley by luring them into the poison. It will most likely kill the monster but the monster will still drop a bag even if you don't have to touch the monster. As soon as the monster moves across the poison, the poison will disappear so you won't have to worry about running into the poison again.