Serious Damage is an item that, when picked up, it doubles the amount of damage the player does for a certain amount of time, depending on the game. Its appearance is based on the logo from the Serious Sam games.
Serious damage can be found in secrets as well as scattered throughout levels. If a serious damage is found normally, then it's usually a big sign that the player is going to fight off a large horde very soon.
Serious damage is a very useful item against hordes of enemies. It is particularly useful when paired with a rapid-fire weapon, such as the XM214-A Minigun or XL2 Lasergun, as it gives them the strength to quickly slice through hordes of enemies.
The player should never use the XPML21 Rocket Launcher or fire grenades while in close quarters, as the resulting splash damage can easily leave the player with very little health left, if not killing them outright.