Complex sentence: Krakens pose a deadly threat in bodies of water where they dwell; they use the Duel spell to prevent wizards from escaping and holding them within range of the Kraken's lightning bolts.
Split into simpler sentences: Krakens pose a deadly threat in the bodies of water in which they dwell; they use the "Duel" spell to prevent wizards from escaping. Krakens can hold wizards within range of the Kraken's lightning bolts.