Why did Lord Vishnu take the Mohini Avatar

• It was the time, when Devtaas and Danavas were confronting a clash against each other after Samudramanthan.
• As the word itself says “Amritmanthan” is churning of amrit or ‘nectar of immortality'. Both devtaas and danavs wanted elixir in their possession for the sake of immortality.
• Dhanvantari appeared infront of devatas and danavas as consequences of Samudramanthan. Deities and demons ran after him to get nectar and eventually, demons snatched away amrit from dhanvantri.
• Disappointed and exhausted devtaas went in for Lord Vishnu's refuge. Vishnu disguised as an enchanting apsara, Mohini. Disguised vishnu went to asuras and using beauty as an instrument to bewitch them to take elixir into his possession. Lord Vishnu successfully overtook amrit from asura's hold, by provoking them to fight with each other for the sake of mohini's beauty.

Mohini distributing ‘nectar of immortality' to devtas while danavas sitting enchanted.

• Later on devtas consumed “nectar of immortality”, this came into reality only and only because of Lord Vishnu and his 'Mohini roop'