text: Rudra is mentioned along with a litany of other deities in RV 7.40.5. Here is the reference to Rudra, whose name appears as one of many gods who are called upon:
  One scholiast's interpretation of the Sanskrit word , meaning 'ramifications' or 'branches', is that all other deities are, as it were, branches of Vishnu, but, Ralph T. H. Griffith cites Ludwig as saying, 'This [...] gives no satisfactory interpretation' and cites other views which suggest that the text is corrupt at that point.
  Non-Rigvedic hymns
  In the various recensions of the Yajurveda is included a litany of stanzas praising Rudra: Maitrāyaṇī-Saṃhitā 2.9.2, Kāṭhaka-Saṃhitā 17.11, Taittirīya-Saṃhitā 4.5.1 and Vājasaneyi-Saṃhitā 16.1–14. This litany is subsequently referred to variously as the Śatarudriyam and the Namakam (because many of the verses commence with the word namaḥ, meaning 'homage'), or simply the Rudram. This litany was recited during the Agnicayana ritual ('the piling of Agni'), and it later became a standard element in Rudra liturgy.
  A selection of similar stanzas is included in the Paippalāda-Saṃhitā of the Atharvaveda (PS 14.3–4). This selection, with further PS additions at the end, circulated more widely as the Nīlarudra (or Nīlarudra Upaniṣad).  Lubin suggests that in the Nīlarudra, lightning is envisioned both as Rudra's arrows and as the deity himself: