The Nihang, like the Warrior Monk, is a military unit with its own special set of promotions - Jangi Kara, Jangi Mojeh, Sanjo, Tegh, and Trehsool Mukh - which must be purchased with Faith and is available only to the Suzerain of Lahore. A Nihang with all five promotions will have 3 Movement, suffer no combat penalties when damaged, receive +7 Combat Strength when flanked and +10 Combat Strength while its owner is Lahore's Suzerain, and award its owner with Faith equal to 50% of the base Combat Strength of each enemy unit it defeats. This means that a properly promoted Nihang will have 77 Combat Strength before any additional modifiers such as terrain, government, flanking bonuses, or other civilization or city-state-specific abilities or bonuses.
The Nihang's abilities make it most valuable to leaders of civilizations that have both religious and military bonuses, such as Chandragupta, Philip II, and Tamar. Alexander, Shaka, and Simón Bolívar also benefit from purchasing Nihang when possible: since these leaders have incentives to build and develop Encampments quickly, their Nihang will reach their full potential sooner than those owned by their opponents.
The greatest strength of the Nihang, however, is its unbelievable scaling into the mid- and late game while keeping its low cost and Gold maintenance. It gains Combat Strength for each new type of Encampment building its owner constructs, allowing it to remain useful until the Information Era rolls around and it gets outclassed by Mechanized Infantry and Modern Armor. The Nihang has two major power peaks: once when the Armory is unlocked (Medieval Era) and once when the Military Academy is unlocked (Industrial Era), at which points you have a unit as strong as but available one era earlier than the standard Musketman and almost two eras earlier than the Infantry that doesn't require any strategic resources. Moreover, Nihang are quite cheap: their cost starts at a base price of 200 Faith and scales according to the tech and civic progression, but can be reduced by adopting Theocracy. For the sake of comparison, a Musketman costs 240 Production and an Infantry costs 430 Production, and the maintenance cost of a Nihang is half as much as a Musketman and a third as much as an Infantry. If you are ahead in Science and can unlock the Military Academy quickly, your opponents won't be able to stop an army of Nihang with strength as high as a unit that comes two eras later, and they barely require any effort to churn out quickly and aggressively.
Lahore should be a priority city-state, as it is dangerous to let it fall into the hands of an aggressive civilization. Try to become its Suzerain, build 1-2 Holy Sites for a bit of Faith generation, build at least one Encampment, beeline for technologies that unlock Encampment buildings and you can roll over absolutely anyone. The AI leaders love building Holy Sites, so by conquering you will generate even more Faith to build up your Nihang army. Remember that Nihang purchased early on will receive their Combat Strength bonuses at the same time as those purchased later, so the sooner you can become the Suzerain of Lahore, the sooner you'll be able to maximize the return on your Faith investment.
Despite being classified as melee units, Nihang don't benefit from Battering Rams, Siege Towers, or Akkad's Suzerain bonus. They do, however, receive Combat Strength bonuses from Oligarchy, Fascism, and Ambiorix's ability and XP bonuses from Encampment buildings, and are boosted by Great Generals of any era.
The Nihang were a sect of Sikh religious warriors in the 17th - 19th century in what is now northern India, Pakistan and Afghanistan. Their name, meaning “crocodile” in Persian, referred to their ferocity in combat, but they called themselves the Akali, literally “the immortals.” Wearing an iconic outfit of electric blue and a turban a foot high, the Nihang were masters of the shastar vidya, Sikh martial arts. In addition to their military skill, they were known for their strict codes of morality and religious discipline, swearing off alcohol, tobacco, and other indulgences; a Nihang, according to a Sikh scholar of the time, is he who “guards the Sikh temples without a desire for material gain, he who is always eager to fight a just and righteous war.” And fight they did; as one official reported, the Nihang were “unaffected by pain or comfort... where there is the place of battle, having no fear of death, he never steps back.”
But, formidable fighters though they were, they were not an army. Rather, they were warrior-priests who would wander the countryside helping themselves to what they needed and defending Sikh communities and temples as they saw fit. The arrival of the British mobilized them into a grassroots resistance movement, one which constantly threatened to sweep out of control. As a result, the leader of the short-lived Sikh Empire, Ranjit Singh, sought to control and co-opt their fierce fighting power when he organized several thousand Nihang into his army around the conquest of Lahore in the early 1800s.
By the mid-1800s, Nihang and the Sikh Empire had gone down in defeat. However, their art is still practiced in some Sikh communities in India and the UK.