Fudō-myōō

In Vajrayana Buddhism, Ācala (alternatively, Achala or Acalanātha (अचलनाथ) in Sanskrit or Fudō-myōō in Japanese) is the best known of the Five Wisdom Kings of the Womb Realm. 

He is also known as Ācalanātha, Āryācalanātha, Ācala-vidyā-rāja and Caṇḍamahāroṣaṇa. 

The Sanskrit term ācala means "immovable"; Ācala is also the name of the eighth of the ten completion stages of the Bodhisattva path (the Eighth Bhumi). 

His siddham seed-syllabe is "hāṃ".

Ācala is the destroyer of delusion and a principal protector of Buddhism. 

His immovability refers to that aspect of mind (Buddha Nature) which is forever unmoved - perfectly stable and unchanging. Despite his fearsome appearance, his role is to aid all beings by showing them the true essence of the teachings of the Buddha, 

leading them into perfect mental discipline.

He is seen as a protector and aide in attaining goals. Shingon Buddhist temples dedicated to Ācala perform a periodic fire ritual in devotion to him.

The buddha Akshobhya, whose name also means "the immovable one", is sometimes merged with Ācala. In most traditions, however, Ācala is not technically a Buddha, but one of the Five Wisdom Kings of the Womb Realm as found in the Japanese Shingon sect of Buddhism. 

Acala is also associated with Dharmapala Vajrapani who is also hailed as the Chief Protector of Buddhism and the seed mantra of Fudo Myo is very similar to the seed mantra of Vajrapani.