The Demon No God Could Kill
Mahisha's boon; the gods thrown out of heaven. The problem that beats every one of your strengths taken singly.
Mahishasura, a buffalo-demon, wins a boon that no man or god can slay him, then defeats the assembled gods and seizes heaven, taking over the offices of the gods themselves. The tradition reads him as the brute ego, the arrogance that beats every one of your strengths taken singly and that no separate power can bring down alone.
Cast Out of Heaven

The gods were wandering the earth like beggars. Indra, who had ruled the heavens, walked among mortals with no throne and no roof. The bright halls of the sky were shut to him and to all the devas, and the one who had taken them was not a god at all. He was a demon with the head of a buffalo, and his name was Mahishasura.
They had fought him. That was the bitter part. The whole assembly of the gods had gone to war against him and lost. He had beaten Indra and the guardians of the eight directions, driven them from the sky, and then done something stranger and worse than simply ruling. He seized the offices of the gods themselves. Where the sun should burn, he burned. Where the wind should blow, he blew. Where the rain should fall, he decided whether it fell. He sat in heaven doing the work of every god at once, and doing it for himself, and the true gods stood on the earth with empty hands and watched a world run by the wrong power.
दुर्गे स्मृता हरसि भीतिमशेषजन्तोः स्वस्थैः स्मृता मतिमतीव शुभां ददासि। दारिद्र्यदुःखभयहारिणि का त्वदन्या सर्वोपकारकरणाय सदार्द्रचित्ता॥
durge smṛtā harasi bhītim aśeṣa-jantoḥ svasthaiḥ smṛtā matim atīva śubhāṃ dadāsi | dāridrya-duḥkha-bhaya-hāriṇi kā tvad-anyā sarvopakāra-karaṇāya sadārdra-cittā ||
O Durga, remembered in danger you take away the fear of every being; remembered by the untroubled you give a mind of great goodness. O remover of poverty, pain, and fear, who but you is ever tender of heart for the good of all?
Devi Mahatmyam 4.17 (Shakradi Stuti)
The Buffalo Who Could Not Be Killed
How had it come to this? Mahishasura had done long and fierce penance, standing through seasons, burning away comfort, until a boon was granted him. And he had chosen his boon with a proud care. Let no man and no god be able to kill me, he asked. He did not trouble to include women in the request. In his mind a woman was no possible threat, not worth the breath of naming. That single gap, left open by his own contempt, is the hinge the whole story turns on, though no one can see it yet.