The World Underground is a book in The Witcher 3: Wild Hunt.
Reading it will give a bestiary entry for ice trolls.
- We hunkered down for the night in a cave so enormous the light from our torches faded into the murky black without revealing any upper bound to it. For most of the night, all we could hear was the rushing of water somewhere behind the wall of the cave. Yet after some time (it is hard to say how much, for we had lost all sense of where day ended and night began) we heard a sort of scraping noise, and then the sound of boulders shifting.
- The scraping turned to a roar, but by that time we were already on our feet and running towards the tunnel through which we had entered the cave. We did so none too soon, for the shifting had opened up a new passage in the cavern, which began to spew out hordes of small, misshapen humanoids. Without breaking stride I looked over my shoulder, and in the midst of the mass of midgets I spied the outline of a large, black troll.
- Thus began our run for our lives.