Coming in the near future. They will be present in Dungeons, and also somewhat rarely in the overworld. They will be somewhat difficult and will require skill to defeat.
There will also be "Ubermobs," unique to each Area generated in the game. These will be much more difficult than normal bosses and feature alignment and diplomacy mechanics. Defeating one will be a huge accomplishment for any group skilled enough to handle it. Ubermobs are more of a long term objective, but have been planned from the very beginning. Planned Ubermobs include Liches, Dragons, Giants, local Monarchs, and more.