Serious Difficulty is a difficulty setting found in the Serious Sam games, it is featured in virtually all Serious Sam titles. Finishing Serious Sam 1 or Serious Sam 3 on Serious Difficulty unlocks Mental Difficulty.
"Maybe we got to beat game... in Serious Difficulty!" - One of the guys from Serious Sam 2 Credits.
Serious Difficulty is generally the penultimate difficulty in most of the Serious Sam games, as Mental Difficulty is usually featured as the last possible difficulty. Only in certain games is Serious Difficulty the highest, and final difficulty setting. Serious Difficulty can be considered the greatest challenge in a Serious Sam game, as it offers highest caliber of enemies featured in any difficulty. Whereas Mental Difficulty, despite being the last possible difficulty in some games, only offers the same amount of enemies featured on Hard Difficulty.
In Serious Sam 1, the player is granted double ammunition collection, and the maximum ammunition capacity. In Serious Sam HD, and Serious Sam 3: BFE, there is also a score multiplier of 50/51. To players seeking further challenges, this difficulty also unlocks Mental Difficulty depending on the game.
Enemies have a drastically increased damage multiplier (x2 in SS1 and SS2, x2.5 in SS3), making dodging and accurate shooting of the utmost importance to ensure survival. A lot of enemies also move faster than in normal difficulty. In addition to this, the amount of enemies is also drastically increased. It is not uncommon to encounter hundreds or even over 1,000 enemies in a single level in the later levels of some games; for example, during the level The Guardian of Time in Serious Sam 3: BFE, well over 2,000 enemies can spawn for you to fight, though it is not necessary to kill all of them. In Serious Sam 2, enemies also have 25% more health.
As a result of the increased amount of enemies, waves of enemies will be much denser, and last longer. Ambushes (caused by picking up things like Extra Small Health) can also feature more enemies than normally found. Larger battles usually receive additional units which attack players far away. In SS1, some of the enemies are replaced by stronger ones as well, most common case being Gnaars and beheaded rocketeers being replaced by kleer skeletons. The amount of enemies can be overwhelming, all it takes is one possible mistake to kill, or heavily damage a player. It is absolutely important to stay alert, as it is very easy to lose track of every enemy on screen, especially when dealing with Kleer Skeletons, or Marsh Hoppers in excessive numbers. In Serious Sam 3, the amount of enemy increase is less drastic with typically only a few additional enemies per level.
Serious Difficulty is recommended only for the serious, experienced players. Hard difficulty might be the closest comparison, but it is by no means an equal equivalent to the overwhelming amounts that can be found on Serious Difficulty. Though obviously not impossible, Serious Difficulty remains a worthy challenge for anybody looking to test their skills. It requires not only good playing skills, but a good endurance from the player. Levels will be much longer due to length and density of waves, so it would not be unusual for first-time players to be playing much longer on certain levels. Players who have knowledge of the levels, and where and when enemies spawn should be more than capable for the challenge.