Famous in the madhwa line, Sri Raghavendra is linked with many miracles. His samaadhi is at mantrAlayam.
Whose avatAr was he ?
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I'd like to start by pointing out that it's not his samadhi but the vrindavana which is present in Mantralaya.
Sri Raghavendra Swami is considered to be the 4th incarnation of Sankukarna, who was an attendant of Lord Brahma. He was designated with the task of providing Lord Brahma with the articles needed for daily worship.
From the book, Sri Raghavendra Teertha - Life history and works.
It was a divine plan. One day Sankukarna was late in bringing the articles of worship to Brahmadeva, who was not happy with the delay caused. A curse outwardly (a boon really) befell SankuKarna to be born as a human being in the mundane world.
A nice read on the life of Sri Raghavendra Swami is Sri Raghavandra Mahatmyam - Saint of Mantralaya, which also mentions this story at the very start:
Shankukarnakya devasthu brahma shapasya boothale
Prahalada ithi vikhyatho bhoobaraksha pane rathara
[...] raghavendraksha yathi rupena sarvadha
kaliyuge ramaseva kurvan mantralaye bhaveth
One of the books which mention the story in detail is Kaliyugada Kalpataru, by Raja Gururajacharya. There is an entire chapter dedicated to this story called "Shankukarna". If you know the Kannada language, you can read it online.