Complex sentence: In a race against time, Chrom performs the Awakening to summon Naga; however, the divine dragon reveals she only has the power to put Grima to sleep for another thousand years.
Split into simpler sentences: In a race against time, Chrom performs the Awakening and summons Naga. Although Chrom now has the power to stop Grima, the divine dragon reveals that she only has enough power to put Grima to sleep for another thousand years.