Powerful beings in Altera can create areas of influence where their power is absolute. These areas are known as Nests and cannot be entered by other beings without permission, though this can be circumvented with specific items that serve as keys.
During the main story players witness and interact with the Nests of the Gold Dragon Gerrant and the Black Dragon Velskud, but more monstrous entities have created their own Nests all over the land. Monsters in these areas are much stronger due to being infused with the Nest Boss's dark powers and are an incredible threat to all but the strongest of adventurers at the same level.
Nests are the final challenge for players who reach the level cap in the update that introduces them. They start at Abyss difficulty and subsequently offer an even fiercer Hell difficulty (not available for the Minotaur Nest) with better rewards. Players will have to run both versions multiple times if they wish to craft the signature equipment and claim the Nest Boss's powers.
Entering a Nest
To enter a Nest, every player in the party must have its associated Nest Pass, which can be obtained from any Dimensional Rabbit's Box. This allows access to the Abyss version of the nest. To access the Hell version, each party member needs the Rare version of the Nest Pass from the Abyss difficulty Reward Chest. Any player who does not have either version of the Nest Pass will find an impassable black miasma blocking the entrance.
Every Nest can only be entered a certain number of times a week. All Nests can be accessed from the portal chamber in North-east Saint's Haven.
Why challenge the Nests?
The main reward for completing a Nest is a variety of materials that are needed to craft that Boss's signature equipment set (one for each class), which has tremendous set bonuses for its level. The final Reward Chest also contains multiple pieces of 'regular' (non-Boss related) Epic equipment, Epic Powders/Extracts/Crystals, Epic Crests and occasionally a Special Skill Crest that allows a player to use one of the Boss's attacks.
Even if the the rewards themselves are unsatisfactory, Nests also offer more gold than any other dungeon and the Boss's unique 'trash' drops can be NPCed for a hefty sum.