Indra is described as having killed Trishiras and in revenge Tvastar created Vritra who was also slayed by Indra. But few verse from the Vedas mention someone else as the slayer of Trisiras:
Rig Ved Verse 10.8.7 states:
7 Through his wise insight Trita in the cavern, seeking as ever the Chief Sire's intention, Carefully tended in his Parents' bosom, calling the weapons kin, goes forth to combat.
8 Well-skilled to use the weapons of his Father, Āptya, urged on by Indra, fought the battle. Then Trita slew the foe seven-rayed, three-headed, and freed the cattle of the Son of Tvaṣṭar.
9 9 Lord of the brave, Indra cleft him in pieces who sought to gain much strength and deemed him mighty. He smote his three heads from his body, seizing the cattle of the oniniform Son of Tvaṣṭar.
Another Verse RV 8.7.24 is extolling Maruts for helping Trita Aptya who was helped by Indra:
24 They reinforced the power and strength of Trita as he fought, and helped Indra in battle with the foe.
The following verse RV 1.187 even mentions Trita as the one who killed Vritra:
- Now will I glorify Food that upholds great strength, By whose invigorating power Trita rent Vṛtra limb from limb.
So who did actually kill Trishiras & Vritra? If it is Trita who is he?
I found who Trita Aptya was in the Shatapatha Brahmana 1.2.3 which mentions the following:
Fourfold, namely, was Agni (fire) at first. Now that Agni whom they at first chose for the office of Hotri priest passed away. He also whom they chose the second time passed away. He also whom they chose the third time passed away. Thereupon the one who still constitutes the fire in our own time, concealed himself from fear. He entered into the waters. Him the gods discovered and brought forcibly away from the waters. He spat upon the waters, saying, 'Bespitten are ye who are an unsafe place of refuge, from whom they take me away against my will!' Thence sprung the Âptya deities, Trita, Dvita, and Ekata.
They roamed about with Indra, even as nowadays a Brâhman follows in the train of a king. When he slew Visvarûpa, the three-headed son of Tvashtri, they also knew of his going to be killed.; and straightway Trita slew him. Indra, assuredly, was free from that (sin), for he is a god.
So according to Shatapatha Brahmana it was indeed Trita who killed Trishiras. The story also helps us make some sense of the earlier Vedic verses regarding him knowing thec 'chief-sire's intention'. But then how do we reconcile this version with the verses that claim it was Indra who performed the act?
Please note I am not looking for Puranic versions only what can be gleaned from the Vedic literature like Samhitas, Brahmanas etc.