The Zen Tree Master is a bonus troop camp that spawns an Essence Master on offense and defense. It was awarded during the Legends of War following the Aftershock! tournament to the top 50 alliances, but was subsequently made available in the Onyx Market for 5,500 onyx. At this price, it is easily one of the best purchases in the game.
The Zen Tree Master is not a standard essence master at level 1, but at level 2 he becomes a standard level 1 essence master, and continues to upgrade until being roughly equivalent to a level 5 essence master. Since he is an Essence Master, he also gains Kami bonuses from the Kami Tree. This makes the Essence master upgrades even more beneficial as owners of the Zen Tree Master will always have an Essence Master in their lineup.
The token to unlock level 5 for the Zen Tree Master was awarded as a prize to the top 50 alliances during the "Legends of Zen" Legends of War tournament held in September 2016. The token to unlock level 6 was awarded as a prize to the winning alliance (only) in that same tournament. Following the tournament, the tokens to unlock levels 5 and 6 are now available for purchase in the Onyx Market.