Rameshwara is located at the extreme southeastern point of India. The sethu bridge was constructed to link this land to Lanka.
The jyotirlinga was worshipped by Lord Rama to atone the sin of killing the demon Ravana. Hanuman flew to bring the linga from Kailasa. But it was getting late so Lord Rama instead worshipped a lingam that was made of sand by his wife Sita. That lingam is known as Ramanathar.
Hanuman was disappointed that Rama had not used the lingam that he brought so Lord Rama pacified Hanuman and named this lingam Kasi Viswanathar. Devotees have to workshop Kasi Vishvanathar before they worship Ramanathar. The waters at the Angni tirtha in the temple complex are considered to have medicinal qualities.