Serious Damage looks like a floating version of the Serious Sam logo. In Serious Sam 1 and Serious Sam HD, it gives the player 40 seconds of double damage, while Serious Sam 2 gives the player 60 seconds. If someone has obtained the power-up, they will have a purple and red glow around them in Serious Sam: The Second Encounter, a lava texture with a fire effect on the user's screen in Serious Sam HD, and a red-liquid-like texture will surround them in Serious Sam 2.
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.
Never use the XPML21 Rocket Launcher while in close quarters, as the resulting splash damage can easily leave you with very little health left, if not killing you outright.
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.