Unique monsters are powerful variants of normal monsters that are randomly generated throughout an area. They can be immediately recognized by their unique name and gold name text. Each unique monster has substantially more life as well as five or six additional modifications that augment its standard abilities, which are chosen randomly from a pool of affixes. All of these modifications can be seen listed below the unique monster's name so that the player can adapt his tactics accordingly.
Unique monsters are particularly dangerous, not just because of they are much tougher than regular monsters. Since they appear randomly, and with random modifications, the player can be caught by surprise. Strategies that are successful against regular monsters may not work against uniques. For instance, a character who can normally stay out of range of a certain monster type and attack from a distance could be caught off guard by the sudden appearance of a unique monster with increased movement speed, that is able to negate the character's range advantage.
This randomness means that players will have to stay alert even in familiar areas, since they never know what could be lurking around the corner.
Although it is possible for a two "unique" monsters to spawn with exactly the same name and statistics, such an event would be extremely rare. It is unlikely that a player will ever see two unique mosters that are the same in every way.