Once their magic was powerful, but when dragons and all the other magical creatures of the world (unicorns, kraken, etc.) disappeared, their power beganto diminish. When the last dragon died, they lost most of their magic. However, their power is returning since Daenerys Targaryen's dragons hatched.
The ancient warlocks in the House of the Undying live on in a semblance of life, preserved by their combined power. They try to hold Daenerys and her dragons there, but she escapes them, setting fire to the place and killing the ancients. The warlocks have vowed to kill her for this, and even went to Pentos looking for her.