As I discuss in this answer, the Pancharatra Agamas are the foundational texts of Vaishnavism. Similarly, the foundational texts of Shaivism are known as the Shaiva Agamas, aka the Shaiva Tantras. One of the Shaiva Agamas is known as the Parakhya Tantra, and it takes the form of a dialogue between the sage Prakasha and his student Pratoda.

In this excerpt from the Parakhya Tantra, Prakasha describes the geography of Jambudvipa, the continent where India is located. (Some people equate Jambudvipa with the entire Earth or even something larger than the Earth, but that's a discussion for another time.). In particular, he says this:

To the south of Ilavrita is the mountain called Nishadha, on which Garuda had been poised to kill the serpent Shesha and was forbidden.

My question is, what's the story of Vishnu's bird Garuda trying to kill Vishnu's serpent Adiseshan?

Now I'm aware that Garuda has a long-standing enmity with snakes, due to the fact that he and his mother Vinata were enslaved by the Nagas (whom I discuss here) who were the sons of Vinata's co-wife Kadru. Garuda even got a boon from Indra that snakes would be his natural food, as described in this chapter of the Adi Parva of the Mahabharata. And Adiseshan is the eldest son of Kadru. But I find it strange that one attendant of Vishnu would try to kill another. Could this story have taken place before one or both of them became attendants of Vishnu?

The only story I know of where Garuda and Adiseshan fight is the story of Arulmaakadal Perumal Vishnu temple in Mahabalipuram. Here is what Wikipedia says:

Once Garuda and Adiseshan (the snake on whom Vishnu rests in his abode Vaikunta) got into an argument as to who was more powerful and devoted to their lord. Finding Garuda a little arrogant, Vishnu went in favour of Adisesha. Worried that Garuda might be angered at this, Adisesha sought the protection of Vishnu, upon which Vishnu asked him to roll himself and hide under him. It is said that as a result of this anger, Garuda never flies over this place.

But is this the same incident as what took place on the Nishadha mountain?

Also, the Parakhya Tantra says "Garuda had been poised to kill the serpent Shesha and was forbidden." So who forbade him? I assume the answer is Vishnu, but I'm not sure.