It is found hanging like an ordinary tree above a Resplendent Chest. The player character, when approaching it, will feel that something is not right (so will the followers), and comment that an unguarded chest is definitely a trap. When the chest is opened, The Old Man and four Gnarled Walkers come to life and attack.
In combat, The Old Man has no affixes, but spawns more sprouts per attack than other Walkers can, and does so much more often. His minions have greatly increased damage and Life. This will result in the entire area around the chest being covered in sprouts in less than five seconds after the combat starts, so kiting the Walkers around is advised.