Rares, Paragons and Artifacts

Rares

 * The world is littered with spawns. Many of these spawns have either obvious or secret bosses, minibosses or things called champions/minichamps.
 * Most, if not all, boss type spawns have a small chance to drop a unique item. Things such as artifact shields/armor, special colored dye tubs or blessed utility items.
 * Farming in a particular spot for an unspecified amount of time can net you a rare spawn. For instance killing a large a mount of youngling monsters might net you a guardian spawn in order to stop the slaughter.

Paragon

 * Paragon monsters have a small chance to spawn in place of the normal mob and are a more powerful version of their counterpart. They'll be colored gold and have a paragon tag under their name.
 * Paragon monsters drop Paragon chests. The chests are locked and correspond with the lock picking difficulty on chests from treasure hunting. These chests can look mundane or extremely unique - great for decoration.

These chests are locked by level of Lockpicking required to open them, from weakest to strongest:
 * Plainly
 * Adeptly
 * Expertly
 * Cleverly
 * Deviously

Artifacts

 * Artifacts usually have unique properties and combinations not found on normal items.
 * Artifacts have a small chance to be part of the loot a treasure chest produces.
 * As stated above most artifacts are attached to boss-type spawns and can be farmed.

Known Artifacts

 * Gargoyle Pickaxe
 * Stimwalt's Shield of Concentration
 * Nystul's Regalia