Durga devi wasn’t created by the devatas. She was already present even before the devatas created her. She is not some devi who was created, but rather the one who created the devatas. The Devi mahatmya or Durga Saptashati says that Mahishasura had the boon of Brahmadeva that he cannot be killed by anyone be it a devata, a demon, animal or anything and should attain death only via a woman. Now why he chose woman? Because he considered them as powerless.
Mahishasura became invincible by the boon of Brahma. He waged a war against the devatas and this war continued for 100 years wherein he defeated the devatas. Now since, Brahmadeva was infinitely superior to the devatas, his boon cannot be overriden by them. So, they got defeated by Mahishasura. Now this Mahishasura took the kingship of the heavens. Seeing this, the devatas were worried and they approached Vaikuntha and sought refuge in Lord Vishnu. Lord Brahma and Lord Shiva were to present there. Lord Vishnu was very angry at this. His eyes went red as he expressed his anger. Same happened with Lord Brahma and Lord Shiva too. Similarly, all the devatas expressed their anger. From each one of them, a light or jyoti came out and merged to form a Devi.
Now, who is this Devi? She is the same Vishnumaya Durga who was present with Lord Vishnu in Yoganidra in his eyes, during the the time of creation. This Vishnumaya Durga devi is present in every devata as their shakti or power. Without her, the devatas are powerless. Same with their weapons too. In every weapon Devi is present. That’s why we say, ‘Yaa devi sarvabhooteshu Shakti roopena samsthita’. So, that devi who was present in Shakti roopa within the devatas came out of those devatas and merged to form her Mahalakshmi swaroopa. But one must note that within Lord Vishnu, she is not present as His shakti but it’s Lord Vishnu who is present as Her Shakti. The Vaamana Purana states that, when Devi went for the battle Brahma asks Devi that why didn’t She wear an armor for the battle? At this Devi replied that Lord Vishnu himself is her armour and she doesn’t require any physical armor.
ब्रह्मोवाच ।
कथमद्य विना वर्मन् समरं गन्तुमिच्छसि ।
को हि नाम नरो वीर सन्नाहं न करोत्युत ॥
How do you wish to go to battle today without armor? What brave person indeed does not put on mail (or prepare a shield) before a fight?
देव्युवाच ।
मम जनार्दनः वर्म पद्मजत्वं न वेत्सि किम् ।
तस्मिन् स्थिते जगन्नाथे किं सन्नाहेन मे फलम् ॥
Devi said, “Lord Janardana (Vishnu) himself is my armor (Varma/ Kavacha). O Lotus-born, do you not know your own origin? When that Lord of the Universe (Vishnu) is present everywhere around me what is the use or purpose of me wearing a material suit of armor?”
Plus, even in Ambhruni sukta, which is known as the Devi Sukta, she says ‘Mamayonirapsvantaha samudre’ which means ‘My origin is the one who sleeps in the ksheerasaagara (Lord Narayana)’. So, Lord Vishnu is the origin of the power of Durga devi and is present within Her as Her Shakti. With His blessings, She is immensely powerful. Now, this roopa which came out was Mahalakshmi form of Durgadevi. She was not created by anyone. She was present within the devatas as their Shakti, but She just came out and appeared in her Mahalakshmi form, that’s it.
Only Durga devi could kill Mahishasura because firstly She was a Kanya (Durga devi is Kanyika and not married to anyone), plus She is Shakti swaroopini within all devatas, humans and demons. And this was as per Brahmadeva’s boon. Lord Vishnu could also have easily killed Mahishasura, but He respected Brahmadeva’s boon. Now, this Devi was holding a roasary, parashu (axe), a mace, an arrow, a thunderbolt, a lotus, a bow, a water pot, a bell, a danda, Shaktyayudha, a sword, a shield, a conch shell, a ritual drinking vessel, a trident, paasha ( a noose), Sudarshana chakra. Like this she had 18 hands and was riding on a lion. Her body was very tall, her crown touching the sky. Mahishasura was awestruck on seeing such a large Devi in front of him and was thinking for a second, ‘What is this?’
Devi easily killed the large troop of Mahishasura. Now, when Mahishasura came, he used to change his forms. Sometimes, he would become a lion, sometime a human, sometime something else. Mahishasura finally became a buffalo. During that transition time between becoming a buffalo and Asura body, Durga or Mahalakshmidevi, jumped and put her feet on him and struck her trident on Mahishasura and killed him. That’s why She is also known as Sairibhamardini or Mahishamardini or Mahishasuramardini.
So, in a nutshell I would like to say that Durga wasn’t created by the devatas. She was already present even before the devatas came to existence. During Pralaya, when Lord Vishnu sleeps in the form of a small child sucking his toe, it’s Durga devi only, who is serving him being the darkness over there. She is present as Shaktiswaroopini in other devatas led by Brahma and Shiva, humans and demons. But for Vishnu, She is not the Shakti, but it is Vishnu who is present as the source of Her power as discussed earlier. Those Shaktiroopas of devi within every devata came out and merged to form the Mahalakshmi or Mahishamardini devi roopa.
Hope this helps!
Jay SriRam!
ShriKrishnarpanamastu!