Complex sentence: After killing the drow priestess, they rescue a dwarven Harper, who tells them of another portal in the mountains, which troops and monsters are planning to use to move into Baldur's Gate, attacking the city from within.
Split into simpler sentences: After killing the drow priestess, they rescue a dwarven Harper, who tells them of another portal in the mountains. He also tells them that troops and monsters are planning to use the portals to move from the mountains into Baldur's Gate, attacking the city from within.