Originally, information of the Invaders was unknown to the Regular Army prior defeating the Iron Sentinel and seeing the Invaders. Morden explains that the Rebel Army and the Martians formed a new alliance to get rid of the Invaders. Both factions attempted to rescue Rootmars from the ocean before the Invaders came to attack. They were excavating the ground in Mission 1 to find the Invaders' base.
True to his word, the Invader King, alongside most of the Invaders, is located deep underground. Reaching there is a large cavern full of organic and fleshy structures. The Invader King itself is situated inside of a dome producing new Invaders. With the help of the Rebel Army, the dome is destroyed, and the Invader King is presumed dead. That is until it beams its eyes up and starts the fight on its own.
The Invader King breathes out various energy blasts. It can also shoot an eye-beam that slowly follows the player. Sometimes its body will light up and produce energy waves that fly in at various directions; the green waves can be destroyed for items.
As the Invader King takes damage, it breathes out a wave that snaps out the squishy platform beneath the heroes and slowly climbs down. Once defeated on the sixth platform, it slips from the walls and takes the heroes into free-fall. The Invader King produces waves that fall in wavy patterns. These waves must be shot at in order to avoid getting hit by them. The Invader King is invulnerable at this state until it eventually falls to its impending death.
The heroes barely survive by hanging on the very edge. The Invader King falls and implodes causing a colossal fireball. It blinds the heroes making them fall to their doom. Depending on the route taken in Mission 4, the heroes are miraculously saved by either Morden or Rootmars.
The player has access to the SV-001 Type-R found in the previous section after defeating the Controller. The Slug is useful for its high jump to launch cannons on the Invader King's face and to dodge its speedy attacks by quickly exiting and entering the Slug. The Vulcan's rate of fire is handy for destroying the waves during free-fall.
The Invader King which its proper name was revealed was introduced in Metal Slug Attack as a playable unit. It serves as the Space Army's Ultimate Boss.
Similarly to the Avatar of Evil, the Invader King has been outfitted with custom sprites of its own in order to be used on the map. As the original Invader King was not meant to be a movable humanoid target like the common enemies before it, it squats on the map, and moves by jumping into the air and landing on the nearest enemy unit, crushing them with its weight.
As with all Ultimate Bosses, the Invader King is deployed onto the nearest enemy unit. It utilizes its mass to attack enemies - charging its hand against close up enemies, leaping and stomping on distant ones, and shooting out numerous lasers from both its mouth and chest. The energy blasts and wave attacks were not incorporated into its attack pattern. Similarly to the original series, it also utilizes eye beams, targeting the unit that caused it to stagger, in addition to a powerful suicide attack when killed. If placed in a deck consisting of Alien units, it also provides an AP-like Fever Time effect.
- The Invader King bears a strong resemblance to Emperor Demon Gyaba, an enemy appearing in most games of Konami's Contra series, regularly as a final boss.
- Prior to Metal Slug Attack's English reveal of the name, it was often referred to by players as the Invader Queen or Venusian Queen. "Queen" stemming from the fact the Invader King is located in some sort of breeding chamber and was assumed it was the matron of the Invaders.
- Monthly Arcadia: Issue 6/2006, pg. 87. http://6th-divisions-den.com/archives/ms6/Monthly%20Arcadia%206.2006/Scans/MA%206.2006%20Page%2086-87.jpg