Complex sentence: In the undersea city of Istar, the sea elves are under threat from an undersea part of the dragon armies; if the "Battlesystem" rules are used, the major underwater battle above and around Istar can be run, for which new rules are given.
Split into simpler sentences: In the undersea city of Istar, the sea elves are under threat from an undersea part of the dragon armies. At this point, if the "Battlesystem" rules are used, the major underwater battle involving Istar can be run, for which new rules are given.