Complex sentence: George sees shimmering colored light in front of him; then he sees what seems to be an enormous dark figure advancing toward him threateningly--the Brocken spectre created by the shadow of Robert sneaking up behind him.
Split into simpler sentences: George sees shimmering colored light in front of him. Then he sees the shadow of an enormous dark figure advancing toward him threateningly--the Brocken spectre created by the shadow of Robert sneaking up behind him.