Complex sentence: When the Knights of Takhisis landed upon their shores, the Irda, enticed by Takhisis, decided to open the gem in order to release whatever magic was inside to protect their lands, inadvertently releasing Chaos, Father of All and Nothing.
Split into simpler sentences: When the Knights of Takhisis landed upon their shores, they decided to crack it open and release whatever magic was inside to protect their lands. When they did break it open though after being enticed by Takhisis, they inadvertenly released Chaos, Father of All and Nothing.