As the Devas were constantly tormented by the Daityas, they went to the sage Kaśyapa, their father, and asked him about help. The sage was concerned about evil actions of Daityas. With a desire to protect the gods, he went to the Kaśi, where made a severe penance with the Lord Śiva as a goal. After that, the Lord Śiva appeared in front of Kaśyapa and the sage asked Him to help Devas in their opposition with Daityas. Śiva agreed and took His birth from Kaśyapa and his wife Surabhi in the form of eleven Rudras (ekadaśa-rudras), who killed a whole generation of Daityas.
These are the eleven Rudras born of Surabhī, Kapālin, Piṅgala, Bhīma, Virūpākṣa, Vilohita, Śāstṛ, Ajapād, Ahirbudhnya, Śiva, Caṇḍa and Bhava. They were born for the aid to the gods. They are identical with Śiva and the cause of happiness.
Along with twelve ādityas, eight Vāsus and two Aśvinas, eleven Rudras make up the Vedic pantheon of thirty-three gods.
Different sources mention different names of the eleven Rudras.
see also: «MANIFISTATION OF RUDRAS»