Complex sentence: The Targaryens had held the island-fortress of Dragonstone as the western-most outpost of the Valyrian Freehold for a century before the Doom destroyed Valyria; in the century after the Doom (the'Bleeding years'), they plotted to expand their holdings.
Split into simpler sentences: The Targaryens had held the island-fortress of Dragonstone as the western-most outpost of the Valyrian Freehold for a century before the Doom destroyed Valyria. For a century after the Doom (the'Bleeding years'), the Targaryens had plotted to expand their holdings.