Complex sentence: Before, they used to consume blood freely, but after the Ten Oaths were established and bloodshed was forbidden, their race came to the verge of extinction and they were forced to resort to a lifestyle more akin to succubi.
Split into simpler sentences: Before, they used to consume blood freely, but after the Ten Oaths were established, they were forbidden to do so. As a result, when their race came to the verge of extinction, they were forced to resort to a lifestyle more akin to succubi.