Actually Brahman (and hence it's actual form) can't be fully understand by our mind and intellect.
According to Kena Upanishad:
II-1. If you think, 'I know Brahman rightly', you have known but little of Brahman's (true) nature. What you know of His form and what form you know among the gods (too is but little). Therefore Brahman is still to be inquired into by you. I think Brahman is known to me.
The qualities like immortal are that of Atman that is described in Bhagavad Gita 2.20 actually comes from Upanishad!
1-II-18. The intelligent Self is not born, nor does It die. It did not come from anywhere, nor did anything come from It. It is unborn, eternal, everlasting and ancient, and is not slain even when the body is slain.
Mundaka Upanishd constitutes some useful verses that describe some qualities or forms of Brahman:
II-ii-2: That which is bright and is subtler than the subtle, and that on which are fixed all the worlds as well as the dwellers of the worlds, is this immutable Brahman; It is this vital force; It, again, is speech and mind. This Entity, that is such, is true. It is immortal. It is to be penetrated, O good-looking one, shoot (at It).
II-ii-9: In the supreme, bright sheath is Brahman, free from taints and without parts. It is pure, and is the Light of lights. It is that which the knowers of the Self realize.
Adi Shankaracharya has many-times cited following forms of Brahman in his Bhashya on Mukhya Upanishads:
- Prajnanam Brahman (Aitareya Upanishad 3.1.3)
- Satyam Jnanam Anantam Brahman (Taittariya Upanishad 2.1.1)
- Vijnanam Anandam Brahman (Bruhadaranyaka Upanishad 3.9.28)
According to Shvetashvatar Upanishad, Brahman is Sarva Vyapaka and Sarva Pakarshmana:
III-11: Therefore, that Divine Lord, being all-pervading, omnipresent and benevolent, dwells in the hearts of all beings, and makes use of all faces, heads and necks in this world.
III-19: Without hands and feet He goes fast and grasps; without eyes He sees; without ears He hears. He knows whatever is to be known, yet there is none who knows Him. They say He is the foremost, the great Infinite Being.
V-14: The sun does not shine there; neither the moon, nor the stars. There these lightnings shine not - how then this fire? Because He shines, everything shines after Him. By His light all this shines.
VI-11: God, who is one only, is hidden in all beings. He is all-pervading, and is the inner self of all creatures. He presides over all actions, and all beings reside in Him. He is the witness, and He is the Pure Consciousness free from the three Gunas of Nature.
Finally, anyway, as mentioned earlier, the true form of Brahman can not be understood by mind and intellect and that's why Brihadaranyaka Upanishad says Neti Neti