The affix tātil comes in the chandas (Vedas) after the words 1. śiva 2. śam and 3. ariṣṭa in the Genitive 6th-Case in construction when the sense is 'he does'.,
The word करः is equivalent to करोति formed by अच् (III. I. 134)
This shows that the construction must be genitive. With a krit-formed word, it has accusative force, as, शिवस्य करः = शिवं करोति ॥
Thus शिवस्य करः = शिवतातिः, So श॑तातिः and अरिष्टतातिः ॥ As याभिः॒॒ शन्ता॑ती॒॒ भव॑थो ददा॒॒शुषे॑ (Rig I. 112. 20) शंताती being dual of शन्तातिः, and meaning सुखस्य कर्तारौ; so also अथो अरिष्टतातये ॥,