The quadruple expansion of the Supreme Lord is composed of Vasudeva, Pradyumna, Aniruddha and Sankarsana. Sankarsana, the fourth expansion, is certainly transcendental, but because His activities of destruction in the material world are in the mode of ignorance, He is known as tamasi, the Lord’s form in the mode of ignorance. Lord Shiva knows that Sankarsana is the original cause of his own existence, and thus he always meditates upon Him in trance. Lord Shiva is in charge of the destruction of the material world. Lord Brahma creates the material world, Lord Vishnu maintains it, and Lord Shiva destroys it. Because destruction is in the mode of ignorance, Lord Shiva and his worshipable Deity, Sankarsana, are technically called tamasi. Lord Shiva is the incarnation of tamo-guna. Since both Lord Shiva and Sankarsana are always enlightened and situated in the transcendental position, they have nothing to do with the modes of material nature–goodness, passion and ignorance–but because their activities involve them with the mode of ignorance, they are sometimes called tamasi.
Source: A.C. Bhaktivedanta Swami Prabhupada (2014 edition), “Srimad Bhagavatam”, Fifth Canto, Chapter 17 – Text 16