Necrosis

"A near perfect viral evolution. The Necrosis is frighteningly fast, strong and deadly. Once destroyed, splits into 3 smaller aspects"

-SAS 4 in-game description

The Necrosis is a new boss zombie introduced in. It is the strongest zombie in the game, with the most amount of base health and a dangerous, ranged attack. Upon dying, it splits into three Spawns, each with a specific resistance to either physical, chemical, or thermal damage.

Tactics
The Necrosis has well-earned its position as the most difficult enemy to face in the game. It is fast, turns quickly, has a hard-hitting and long range melee attack, and possesses an omni-directional attack that hurls three different kinds of projectiles in all directions. Strafing to avoid these projectiles is imperative as they come with debilitating status effects as well as a good deal of damage. It is advised to keep a good distance from the Necrosis and pound it with heavy weaponry, remembering to side-step the waves of projectiles.

It should be noted that the Necrosis gets stronger with each level up. It's omni-directional attack also gains more projectiles at certain levels. For instance, at the earliest possible level a Necrosis can appear in Singleplayer Mode, its omni-directional attack is a twelve-shot burst in all directions. At higher levels, the twelve-shot burst becomes a fifteen-shot burst, then an eighteen-shot burst, then finally a twenty-one shot burst, making the projectiles harder to avoid as the player levels up.

However, the fight is not over when the Necrosis is killed. On death, it splits into three weaker, but still dangerous Necrosis Spawns, each possessing different attacks and resistances. One Spawn of each color (red, blue, and green) is always created on the death of a Necrosis.
 * The Blue Spawn is highly resistant to Physical attacks, and possesses teleportation abilities to reach players before they can run to get distance. Because the Blue Spawn teleports, it often attacks most players by surprise, almost always inflicting damage after it teleports.. If not killed quickly enough, the Blue Spawn will take players down, hiding or not, one by one, resulting in a rage-quiting mission failure. It is best to take out this spawn first, since it can catch up to any player regardless of movement speed.
 * The Red Spawn is highly resistant to Thermal attacks, and leaves an extremely deadly trail of fire wherever it walks. Although it presents a mobility hazard, it is arguably the least worrisome of the three aspects if the players were to watch their feet. The red Spawn, like the other Necrosis Spawns, has a melee attack, but the red Spawn's melee attack is like that of a zombie with Fiery Skin, setting the player on fire on contact.
 * The Green Spawn is highly resistant to Chemical attacks. It spits acid, much like a Spitter, frequently, and on its death, it releases a deadly cloud of miasma, and can inflict heavy acid damage with its melee attacks.

Trivia

 * The name of this zombie comes from a medical condition called "necrosis", in which tissue breaks down due to the destruction of cell membranes. Necrosis can be fatal if it is not treated fast enough. It is caused by a number of diseases, as well as various cytotoxins (poisons that attack tissue), such as the one in the bite of the Violin Spider.