The Alwars (also spelled Azhwars) are a group of 12 ancient Vaishnava saints who lived in Tamil Nadu and are famous for their poetry in praise of Vishnu. The Alwars are crucially important figures in the development of Vaishnavism; it is the principles and beliefs embodied in the Alwars' poems that ultimately gave rise to the Sri Vaishnava sect (of which I'm a member). The Alwars praised many Vishnu temples and other sacred places of Vishnu in their poetry; a sacred place of Vishnu that was praised by at least one Alwar is called a Divya Desam, and there are 108 of them;here is the list.
One of the 108 Divya Desams is Thiruvidanthai near Chennai, the site of the Lakshmi Varahaswami temple, AKA the Nithya Kalyana Perumal Vishnu temple:

This is where Vishnu's incarnation Varaha the boar appeared to the sage Galava and married his 360 daughters, all of whom were incarnations of Vishnu's wife Lakshmi. They all merged into a single woman, Komalavali, who appears on Varaha's lap above.
But this web page says that Vishnu also appeared to another sage in this location; for Thiruvidanthai it says "Prathyaksham to Markandeyar". Prathyaksham is a term for a god appearing directly before someone's eyes. So my question is, what is the story of Vishnu appearing before the sage Markandeya in this place?
For those who don't know, Markandeya is a famous sage who was saved from Yama god of death by Shiva because of his devotion to Shiva. But Markandeya is also a great devotee of Vishnu; the famous Upillaiappan Vishnu Temple near Kumbakonam is due to Vishnu appearing before Markandeya in his Venkateshwara form and marrying Markandeya's daughter. But does anyone know the story of Vishnu appearing before Markandeya in Thiruvidanthai? Is this before or after his interactions with Upillaiappan?
- There is a temple by name Adi-Narayana Perumal (also known as Adi-Ranganatha Swami, Adi-Arangan Temple) that is somewhere off Vizhuppuram, and I believe this is the wikipedia page for it, though I am not sure about the accuracy of wikipedia: https://en.wikipedia.org/wiki/Adhirangam_Ranganathaswamy_temple
– Vidyarthi Jan 19 '20 at 01:20Another one is - https://tamilnadu-favtourism.blogspot.com/2018/08/matsya-moorthy-temple-thuvarimaan-legends.html
Look at the first para of that link.
There are other temples ofcourse, but they dont claim to be the avatara-stalam of Sri Matsya Murthi
– Vidyarthi Jan 19 '20 at 01:26But The Puranas such as Matsya Purana & others don't sound like this avatara happened in what is called TamilNadu today. If my memory serves me well, I think Matsya Purana says only the river Narmada existed at the time of Pralaya when Matsya Murthi took people to safety. In Narayaneeyam, Sri NarayanaBhattar describes this avatara as something that happened elsewhere (take this as reference & I will post the exact slokha later)
– Vidyarthi Jan 19 '20 at 01:32