Mahendra Parvata, the Mahendra mountain, is the sacred mountain where Paraśurāma lives in unbroken tapas (austerity). Paraśurāma is a cirajīvin (one who lives for ages) of the Bhṛgu line. In the Kalki Purāṇa the mountain is where Paraśurāma became the guru of Bhagavān Kalki. In the Vālmīki Rāmāyaṇa it is the great mountain on the southern shore of Bhārata from which Hanumān leapt to Laṅkā.

Paraśurāma’s home

Paraśurāma made twenty-one campaigns that cleared the earth of kshatriyas. He performed the Aśvamedha at Syamantapañcaka and gave the earth to the brāhmaṇas. Then he retired to Mahendra Parvata to practise tapas for ever. The mountain is his home for the rest of the present cosmic cycle.

Paraśurāma is one of the seven cirajīvins, so he is still alive on this mountain. His tapas keeps renewing his powers and his awareness. He waits for the moment when his next task calls him.

That moment came with the descent of Bhagavān Kalki. From his place on Mahendra Parvata, Paraśurāma saw the avatāra setting out from Śambhala to find a gurukula.

The Kalki Purāṇa tells this in Prathamāṃśa, Chapter 3.

The gurukula of Bhagavān Kalki

Paraśurāma met Kalki on the way, while Kalki was looking for a teacher, and declared himself Kalki’s guru. He said of himself: “I am Jamadagni’s son of the Bhṛgu line. I know all the Vedas with their aṅgas. I am a master of the Dhanurveda. I cleared the earth of kshatriyas twenty-one times. I retired here to Mahendra Parvata for tapas. Now come and learn from me.”

Kalki bowed before Paraśurāma and accepted him as his ācārya (teacher). The teaching at Mahendra Parvata included:

The four Vedas with their six Vedāṅgas: śikshā, kalpa, vyākaraṇa, nirukta, chandas and jyotisha.
The Dhanurveda, the whole science of archery and war.
The sixty-four arts (catushshashṭi kalās).

When the teaching was complete, Kalki asked what he should give as guru-dakshiṇā, the offering due to a teacher. Paraśurāma showed that he knew who Kalki was. He said: “You are Vishṇu himself. My reward is to witness your divine mission.”

Kalki received all this training from Paraśurāma, the great living warrior and scholar of the age, on the mountain of the Bhārgava line. The Kalki Purāṇa tells this in Prathamāṃśa, Chapter 3.

Mahendra in the Sundara Kāṇḍa

Mahendra Parvata has an important place in the Sundara Kāṇḍa of the Vālmīki Rāmāyaṇa. It is the great mountain on the southern shore of Bhārata. From it Hanumān made his leap over the ocean, a hundred yojanas wide, to Laṅkā.

The vānara search party gathered on the mountain under Aṅgada and Jāmbavanta. They waited there anxiously through the many days of Hanumān’s mission in Laṅkā.

When Hanumān gathered his divine energy for the leap, the mountain shook under his weight. Its rocks broke apart and its trees fell. When he came back, he roared from far away as he came down towards the mountain. Jāmbavanta knew at once that this roar was the sound of success.

The whole search party met again on Mahendra. From there they went on to the Madhu Vana and then to Kishkindhā, with the news that Sītā had been found.

Sources

Kalki Purāṇa, Prathamāṃśa, Chapter 3
Vālmīki Rāmāyaṇa, Sundara Kāṇḍa, Chapters 1 and 57