In the Vishnu Sahasranama, it is the Narasimha avatara that is mentioned first among all the avataras.
Narasimha came heeding the calls of a child, and that too the child of an asura.
It is said that Lord Narayana came as Narasimha for the sake of Prahlada. But a better way of putting it would be to say that He came in order that Prahlada's words might be proved true. The Lord is always true to His words. He never slips from them. Nor does He let his devotees' words prove false. Prahlada had declared to his father Hiranyakasipu that the Lord was there everywhere. That is why the Lord took His position in all pillars, for He wanted to be at hand, whichever pillar Hiranayakasipu chose to strike. He wanted to show that He would never allow His devotees' words to be proved false.
Whatever form pleases His bhaktas, is a form that He readily assumes, and in doing so, the Lord loses none of His auspicious qualities. His form remains always pure and untainted.
Nammazhvar shows this to us, by saying that He is the One who is on earth, heaven and also in the space between heaven and earth. He is one and the same everywhere, with no dilution in His qualities. His form is pure everywhere and at all times. He is unmatched and Supreme, and yet simplicity is His defining quality. Else would He hasten to the side of Prahlada, in His half-man, half-lion form? – Swami Desika
‘This All-Merciful Lord (present before me - Narasimha). Who is Self-same with His Blissful Nature does not accept any worship of a pseudo-devotee but He covets it for the sake of the devotee himself. For whatever honor the devotee offers to the Almighty Lord, it rebounds upon himself, (even) as the decoration of one’s face is (immediately) reflected on its image (in a mirror) which cannot be decorated otherwise’. – Prahlad in Srimad Bhagavad Purana
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A Quiz on the Narasimha Incarnation of Lord Vishnu