Nericon is a powerful tree located in the Mardaneus Plaza district of Aleroth during the events of Divinity II: Flames of Vengeance. Within its branches dwells the soul of an ancient wizard king, who comes forth through vessels of nature to fight undead plagues wherever they arise.
When you first approach the gate separating Crow's Nest from Mardaneus Plaza, you witness Nericon striking down a Champion with lightning. Cautiously approaching the tree begins a conversation, which you initially do not understand. Finding a way to communicate with it is the objective of the quest Bark up the Right Tree. To do so, you'll need the help of book titled Elven Alphabet (found in the Forbidden Archive at the Prancing Seahorse on Lanilor Lane), which allows you to translate the tree's ancient Elven language.
You learn that the tree is dedicated to destroying the taint of undead that inflicts the city, and it serves as a barrier - it will not allow anyone or anything to pass, be it human or undead. However, if you state that you intend to destroy the source of the undead, it will allow you to pass into Crow's Nest unharmed. This grants the "Oaken Resolve" achievement, and begins the quest Nericon's Wrath.
If Nericon is mindread for 22500 exp, it gives you the Dragon Wizard Cuirass, a piece of the Dragon Wizard armor set: "Perhaps you search my thoughts for power, or wealth. How about I grant you the Dragon Wizard's Armour?"