A fierce राक्षस (giant). It is stated in महाभारत , आदि पर्व , Chapter 65, Stanza 22, that this giant was the son of प्रजापति कश्यप by his wife दनु . This fierce giant was killed by इन्द्र . There is a story in the पुराणस् describing how नमुचि was killed.

Under the leadership of नमुचि a great army of the giants invaded the realm of the gods. इन्द्र came with an army of देवस् . Though the giants were defeated in the battle इन्द्र was not able to kill नमुचि . To save himself from the attack of इन्द्र , नमुचि got into the radiance of the सुन् and hid himself there. इन्द्र found him out and made a treaty with him, the conditions of which were as said by इन्द्र :--

Oh, noble giant, I will not kill you by wet thing or dry thing, in the night or in the day. What I say is true.

According to this treaty it became impossible for इन्द्र to kill नमुचि either with wet things or with dry things and either in the day time or in the night. Only when इन्द्र had agreed to these conditions did नमुचि come out.

The battle continued and Indra drove away शुम्भ and निशुम्भ the elder brothers of नमुचि , who went to पाताल . Indra ran after Namuci and in the evening Indra found him hiding on the sea shore and killed him with the foam of the sea. As Indra had violated the condition of the treaty the severed head of Namuci followed Indra. With this Indra incurred the sin of ब्रह्महत्या (killing a Brahmin). To get remission from this sin Indra approached ब्रह्मा . He was advised to bathe in अरुणासङ्गम and doing so Indra got remission of his sin. From that day onwards Aruṇāsaṅgama became a holy bath (tīrtha). [mh-9-40-1 ] Mahābhārata • Śalya parva null .

An army-captain of हिरण्याक्ष . In the battle with इन्द्र , नमुचि made him unconscious and the elephant ऐरावत thrust its tusks on the ground. After that by his magic and sorcery he created many creatures. But विष्णु destroyed all those creatures with his discus सुदर्शन . At last इन्द्र killed नमुचि . (Padma Purāṇa, Sṛṣṭikhaṇḍa)

Another valiant captain of हिरण्याक्ष . He sent five arrows against इन्द्र in a fierce battle. But इन्द्र cut all the five arrows in the midway. Then by his magic and sorcery नमुचि spread darkness everywhere. इन्द्र defeated that strategy also. Then नमुचि dashed forward and taking hold of the tusks of ऐरावत shook इन्द्र down. इन्द्र stood up and cut off the head of नमुचि with his sword. (Padma Purāṇa, Sṛṣṭi Khaṇḍa) .