Who Tvashtri is
Tvashtri is the craftsman among the Vedic gods. His name means the fashioner, the one who cuts and shapes. The Rigveda has no hymn that is his alone, but it mentions him about sixty-five times, and nearly always with his hands at work.
What he makes is the best of its kind. He forged the vajra, the thunderbolt with which Indra killed the dragon Vritra. He sharpens the iron axe of Brahmanaspati. He made the cup from which the gods drink Soma, and the hymns call Soma itself "the mead of Tvashtri", for it is kept in his house. The poets call him the most skilful of workmen, the one who knows every craft.
He also shapes living things. One verse says he adorned all beings with their forms. Another says he shaped a husband and wife for each other while they were still in the womb. People prayed to him for children, and the wives of the gods are his usual company. Because every form is his work, the hymns call him Vishvarupa, the one who holds all shapes.
His family is small and troubled. His daughter Saranyu was given in marriage to Vivasvat, the Sun, and became the mother of Yama; one hymn says the whole world came to the wedding. His son, also called Vishvarupa, had three heads and kept a herd of cattle. Indra killed him.
Indra and Tvashtri are never quite at peace. The same god who forged the thunderbolt had his Soma taken by the newborn Indra, who "overcame Tvashtri" on the day of his birth and drank in his house what he had not been offered. The Ribhus, three mortal brothers, took his one cup and made four of it, and one hymn says he went and hid among the wives of the gods and spoke of killing them.
The hymns do not explain him. Scholars have guessed that he was once a god of an older generation, or an old name for the creative side of the sun. The Rigveda itself only shows the workshop.
For Indra, Tvashtri forged the thunderbolt, the best of all his work, a thing of heaven made for the battle.