This form of Lord Ganesha is considered the 'Shivabrahma' form. As the name suggests, in this form, Lord Ganesha is of a smoky hue and expands His Leela. Lord Dhumravarna incarnated to vanquish Abhimanasura. According to the Mudgala Purana, once the creator of the universe, Lord Brahma, appointed the radiant Lord Surya as the Karma Phala Pradata, i.e., dispenser of karmic fruits. This led to a slight ego rise in Lord Surya Deva's mind. He thought, "Through the influence of Karma, Lord Brahma creates the universe, Lord Vishnu sustains it, and Lord Shiva is capable of destruction. By the fruits of Karma, Shakti becomes the caretaker and nourisher of creation. This entire world is governed by Karma, and I am the deity who oversees all these Karmas. From this perspective, all deities are subordinate to me."

While contemplating, Lord Surya Deva suddenly sneezed, and from this sneeze emerged a mighty, large-eyed, and colossal man. This handsome figure approached the wise Guru Shukracharya. When Shukracharya inquired about his identity, the being replied - "I was born from the sneeze of Lord Surya. I am entirely without support on this earth. I wish to remain under your guidance and follow every command given by you." After meditating for a while, Shukracharya said - "You were born from the ego of Lord Surya Deva, so your name shall be 'Aham' (Ego). You must undertake penance to gain strength."
Following this, Shukracharya imparted the sixteen-syllable Mantra of Lord Ganesha to him. He then went to the forest to perform intense penance. Enduring heat, cold, and rain, he chanted the divine Mantra given by his Guru for a thousand celestial years. Pleased by his devotion, the compassionate Lord Ganesha, adorned with His Mushaka-Vahana, three eyes, elephant face, single tusk, fan-like ears, and various divine weapons, appeared before him. Ahamtasura worshipped and praised Lord Ganesha with heartfelt devotion. Lord Lambodara was satisfied and asked him to request a boon. Ahamtasura then asked - "O Lord, grant me your devotion, fulfill all my desires, bestow upon me an infallible weapon, and grant me dominion over the entire universe. Let my death not occur through any illusionary means." Lord Ganesha granted him the boon with "Tathastu" and disappeared.
After receiving the boon from Lord Ganesha, Ahamtasura returned to his city and narrated the details of his penance and the boon to Shukracharya. Listening to this, Shukracharya was immensely delighted. He gathered all the demons and Daityas and praised Ahamtasura's greatness, leading them to accept his leadership. Shukracharya then anointed Ahamtasura as the king of the Daityas. A grand celebration was held, and a magnificent city named 'Vishayapriya' was constructed for him to reside in. Later, Pramadasura arranged the marriage of his daughter Mamta to King Ahamtasura. Over time, Mamta gave birth to two sons named Garva and Shreshtha.
One day, Pramadasura said to his son-in-law Ahamtasura - "You have received the boon of universal victory and fearlessness, so why waste your time? Go and spread your victory banner across the Trilokas." Inspired by his father-in-law's words, Ahamtasura embarked on a journey to conquer the universe, causing chaos everywhere. The demons wreaked havoc, and Ahamtasura gained control over the entire seven-continent Earth. Subsequently, he proceeded to the Patala and brought it under his control as well. After conquering Patala, he launched a fierce attack on heaven. Using the infallible weapon granted by Lord Ganapati, he defeated all the Gods, including Lord Vishnu, who could not stop him. To escape Ahamtasura's influence, the Gods, sages, and righteous people hid in mountain caves. Ahamtasura indulged in alcohol and meat and shamelessly abducted and violated the daughters of humans, Nagas, and Gods. Due to these sins, he forgot Lord Vighnaraja, the remover of obstacles.
One day, an Asura named Adharmadharaka appeared in Ahamtasura's court and said - "O King! Although you have established control over the entire world, the immortal sages, saints, and Gods are still hiding in forests and caves, working towards our complete annihilation. They could destroy us at any moment. Therefore, efforts must be made to eliminate them. The immortals are nourished by rituals like Yajnas. Once these rituals cease, they will perish on their own." Following Adharmadharaka's advice, Ahamtasura began disrupting Yajnas and other sacred rituals. The Varna system and Ashrama Dharma were shattered. The wicked demons started destroying mountains and forests to cause immense suffering to the Gods. Ahamtasura expelled Lord Ganesha's idols from temples and began worshipping himself. This greatly pleased Adharmadharaka.
All the Gods gathered to deliberate on this grave issue. A distressed Lord Brahma addressed them, saying - "Ahamtasura, intoxicated by the boon granted by Lord Ganesha, is wreaking havoc. In such a situation, we must worship the compassionate Lord Vighneshvara so that He may be pleased and relieve us of our suffering." For nearly a hundred years, the Gods worshipped Lord Ganesha using His one-syllable Mantra. Pleased by their devotion, Lord Vighneshvara, in His Dhumravarna form, appeared before them. The Gods, overjoyed, bowed at Lord Dhumravarna’s feet and performed a formal hymn of praise. They prayed - "O merciful Lord, please remove our calamities." Lord Dhumravarna granted them the boon with 'Tathastu' and disappeared. The Gods then awaited the right moment to act.
That night, Lord Dhumravarna appeared to Ahamtasura in a dream. Seeing the radiant and fierce form of the Lord, the demon was terrified. He shared the details of his dream with his fellow demons, describing how Lord Dhumravarna, in His wrathful demeanour, destroyed the city of the demons and liberated the Gods. However, Ahamtasura's courtiers reassured him, reminding him that he had received the boon of fearlessness from Lord Ganesha Himself, and thus, there was no reason to worry. They dismissed the dream as meaningless, as victories, defeats, and deaths seen in dreams are often deemed false. Ahamtasura, convinced by their reasoning, laughed off the dream. Later, Lord Dhumravarna sent Sage Narada as a messenger to Ahamtasura, urging him to seek refuge in Lord Ganesha. However, this message only enraged Ahamtasura further. The Gods once again prayed to Lord Dhumravarna, who assured them - "Witness my Leela; I will surely slay Ahamtasura." Lord Dhumravarna then unleashed His divine noose, which began a fierce annihilation of the demons. Despite their desperate efforts, the flames of the noose consumed them all. Lord Dhumravarna also killed Ahamtasura's sons, which drove the demon into a furious rage. He entered the battlefield and used all his infallible weapons, but they proved ineffective. Astonished, he fled the battlefield and went straight to Shukracharya. He questioned the failure of the infallible weapons granted to him as a boon. Shukracharya explained - "The words of Lord Ganesha, who transcends illusion, are never false. He ensures the smooth functioning of the lives of Gods, humans, and demons. You have caused suffering to the Trilokas using His boon, and this has angered Him. If you wish to save your life, seek refuge in Him."
Terrified, Ahamtasura began praising Lord Dhumravarna's divine noose. This act calmed the noose, which returned to its master, Lord Dhumravarna. Ahamtasura then fell at Lord Dhumravarna's feet. Pleased by his worship and praise, Lord Dhumravarna granted him devotion and said - "O great demon! Where my worship does not take place in pilgrimage sites and temples, you may partake of the offerings and create obstacles due to your demonic nature. Now, return to your city and always protect my devotees."
Thus, the Gods and all three realms were liberated from Ahamtasura's tyranny. With the re-establishment of Vedic Dharma, the universe resumed its orderly functioning. Finally, everyone prayed to Lord Dhumravarna, the remover of obstacles, and received His divine blessings.