The final vowel of viśva is lengthened before vasu and rāṭ (the form assumed by rāj).,
Thus विश्वावसुः, विश्वाराट् ॥ The rule applies to the राट् form of राज in the Nominative singular, and not when it retains its own form: as विश्वराजौ, विश्वराजः ॥,