Necrosis

The Necrosis (also called Necrosii, Necroses, or Necrosises in plural or just simply "Necro(s)") is a new boss zombie introduced in. It is the strongest zombie in the game, with the highest base health and a dangerous, ranged attack which launches various projectiles in all directions, also possessing energy resistance, which is not given to its spawns. Upon dying, it splits into the three aforementioned Spawns, each with a specific resistance to either physical, chemical, or thermal damage.

Tactics

 * The Necrosis has a well-earned reputation 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.
 * Another is to have one player run from the Necrosis, and have it chase them while the other player(s) pound it with heavy weaponry, turrets and abilities. The player(s) should switch the runner so the Necrosis doesn't catch and kill players, as it will catch you eventually. Flanking and damage over time are the best tactics to use, entire weapon sets and builds have evolved from this tactic. One such combo is a Poison Claw,  HIKS S300, and Luftplatzen, all augmented with deadly and tenacious for maximum damage. Players should use explosives and/or Aerial Bombardment or Zombie Antidote on the spawns once the Necrosis splits after death, so as to get off some free damage at the start on all of them, and in some cases kill one or two.
 * It should be noted that the Necrosis gets stronger with each level up. Its omni-directional attack also gains more projectiles at certain levels. For instance, at the earliest possible level a Necrosis can appear in single-player Mode (rank 39), 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. Upon 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 upon the death of a Necrosis.
 * The Blue Spawn is moderately 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-quitting 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, and that their weapons are not thermal weapons. 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. Rest assured, the damage of the Red spawn is still large, and the fire trail is devastating, as it can kill a player with full health.
 * The Green Spawn is highly resistant to Chemical attacks. It spits acid, much like a super-powerful Spitter, frequently, and can inflict heavy acid damage with its melee attacks. While it doesn't deal as much damage as the Red Spawn, it can still deal high damage, and can easily knock out players without sufficient toxic resistance or health.
 * Having a high level assault team or another player with one (this happens fairly often in co-op mode) will often tank the Necrosis long enough for it to be killed, then the spawns can be killed with the remaining assault members tanking them. Using energy weapons to kill the spawns is a second tactic since the spawns don't have energy resistance, while the default necro does.

Trivia

 * The Necrosis possessed a ranged tentacle attack during the closed beta of SAS 4 (YouTube video), apparently before the Necrosis' current blade, fire, and acid shot attack was implemented. Although it was removed due to a negative response, the animation for the attack still exists in one of the SWF files used by SAS 4.
 * 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 Brown recluse spider. This explains the rotten & highly infected state of Necrosis's appearance in game while still retain some humanoid aesthetic abeit being quadrupedal/
 * "Necro" is also a Greek prefix meaning death. This brings up some possible theories about Necrosis's origin: Necrosis was created by injecting parasite or the virus in an already-dead host, thus making Necrosis being completely controlled by said parasite. This is likely what happened to most zombies in the series as they could be either reanimated corpses or infected while still alive.
 * Both new and veteran Players of the SAS: Zombie Assault franchise have deemed the Necrosis as the most overpowered enemy in SAS 4, not by its initial form but by its blue spawn, which has helped maintain the Necrosis' infamous reputation.
 * Energy weapons are the best option for dealing with Necrosis spawns, as they collectively have no energy resistance. However, the Necrosis itself is resistant to energy attacks.
 * The Savage Necrosis resembles Venom from Marvel comics; its now absent tentacle attack also resembles one of Venom's methods of attack.
 * The Savage Necrosis is slightly larger than its regular counterpart.
 * The necrosis's spawn will have the same virus vial value as bloaters
 * The necrosis can be related to the country road creature, a cryptid created by trevor henderson.