In the Prashnopanishat 2.9 we read:
इन्द्रस्त्वं प्राण तेजसा रुद्रोऽसि परिरक्षिता । त्वमन्तरिक्षे चरसि सूर्यस्त्वं ज्योतिषां पतिः ॥९ ॥
9 Indra thou art, O Prana and Rudra, too, in prowess. Thou art the Protector. Thou movest in the sky; thou art the sun, the lord of lights.
इन्द्रः परमेश्वरस्त्वं हे प्राण तेजसा वीर्येण रुद्रोऽसि संहरञ्जगत् । स्थितौ च परि समन्ताद्रक्षिता पालयिता परिरक्षिता त्वमेव जगतः सौम्येन रूपेण । त्वमन्तरिक्षेऽजस्रं चरसि उदयास्तमयाभ्यां सूर्यस्त्वमेव च सर्वेषां ज्योतिषां पतिः ॥९ ॥
//O prANa, you are Indra, the supreme Lord. By valour you are Rudra, engaged in destroying the world. Again during the time of the existence of the universe, you, in your benign aspect, are the preserver of the universe on every side. You move for ever in the sky by rising and setting are the sun, the lord of all the luminaries. //
Here, along with the other mantras in this section, we see the Upanishad is teaching that One Brahman alone is taking the forms of all devatA-s and engaged in the cosmic administration. Here we get a glimpse of the Upanishadic concept of TrimUrti, the Three Forms engaged in Creation, Sustenance and Destruction of the universe. Rudra, Shiva, is shown as the entity engaged in the destruction function. The Sustainer deity too is another form of the Supreme prANa alone.
While Anandagiri adds, ‘by the forms of ViShNu, etc…’ (which is not found in the BhAShyam, however), the editor Sri Subrahmanyam Sastri feels it is Rudra alone that is the sustaining principle too in his sowmya form. Generally the creation principle is allotted to BrahmA who represents the rajas shakti. In any case the Upanishadic teaching of the One Principle alone that is engaged in the various functions is unmistakable.
Om Tat Sat