The Warrior Monk is a Medieval Era unit introduced to Civilization VI in the Fall 2017 Update. It can only be purchased with Faith in a city that has a majority religion and a Holy Site with a Temple (or one of its replacements).
The Warrior Monk is a military unit with its own special set of promotions: Cobra Strike, Dancing Crane, Disciples, Exploding Palms, Shadow Strike, Sweeping Wind, and Twilight Veil. A fully promoted Warrior Monk will have 60 Combat Strength and 4 Movement, be able to attack twice per turn with double flanking bonuses, be invisible to non-adjacent enemy units, and spread its civilization's religion to nearby cities each time it defeats an enemy unit. Moreover, it's rather cheap to purchase with Faith, making it easy to field an army of Warrior Monks in short order. They're particularly useful to leaders of civs that have a combination of religious and military bonuses, such as Chandragupta, Philip II, and Tamar.
Although they are melee attackers, Warrior Monks are not considered melee units. As military units, however, they can be combined to form Corps and Armies once the appropriate civics have been discovered.
One example of a military policy card whose effects can boost the effectiveness of Warrior Monks (and vice versa) is Raid. When used with Warrior Monks who have the Twilight Veil and the Dancing Crane promotions (they will have picked up Disciples and/or Exploding Palms along the way), the result will be highly mobile and cost effective (if not downright terrifying) raiding units who the enemy will have a hard time tracking, much less stopping. Remember though, they are by no means invulnerable!
Civilopedia entry Edit
The myths of wall-running, magic-palmed warrior monks endure because of how outlandish they are. Unfortunately, we simply have to live with the actual feats of history's priests with astounding fighting skills.
Consider the Shaolin Monastery in China, founded by the Indian Buddhist monk Batuo in 495 CE. While the history of martial training at Shaolin is tangled with legends and pop culture, it's said that Batuo and his first Chinese disciples, potentially soldiers before joining the monastery, already came equipped with martial arts skills, passing them down to their students.
By 581, monks at Shaolin would have their own fighting style, even siding with would-be second Tang Emperor Li Shimin during the dynastic wars of the period.
By contrast, Japan's Buddhist sohei would engage in the less glamourous inter-temple turf wars, picking up arms and using their fighting skills in order to gain political, not spiritual, dominance.