ToplineTexas oil and natural gas firm Diamondback Energy announced Monday it will buy its privately held competitor Endeavor Energy in an agreement valuing the latter at roughly $26 billion, the latest massive energy deal amid an otherwise quiet time for dealmaking. Diamondback and Endeavor, two leaders in the oil-rich Permian Basin, will combine to form a more ... [+] than $50 billion entity.AFP via Getty Images Key FactsDiamondback’s offer of roughly $17.8 billion worth of its publicly traded stock at Fr ...