The assaultron robot was constructed by RobCo and sold to the US Military as a front-line wartime combatant. It is fast and deadly at close range, and employs a devastating laser at a distance. The dominator variant can enable stealth technology.
The assaultron's body seems to be modeled after that of a female figure. They are highly aggressive, fast, and some will attack the Sole Survivor on sight. The assaultron's face will become surrounded with red electric energy when it is preparing its laser beam attack. The dominator variant is also equipped with a stealth field.
An assaultron's laser can be halted or avoided by using Jet and stepping out of the way, killing it, or doing enough damage to its head. If it is allowed to charge its laser all the way, and if not blocked by an object, it will inflict devastating amounts of damage on its hapless target, often killing even high-level players or NPCs in seconds. The fact that assaultrons take reduced damage from headshots only serves to compound their main weapon's threat potential.
Assaultrons' weak points are their legs. However, when these are destroyed, the assaultron will continue to pursue the target by crawling on the ground. After taking sufficient damage, they will charge a potentially deadly explosion as a last desperate attempt to eliminate the target. Killing them before they can activate it will result in a normal death.
Legendary assaultrons will invariably detonate violently, similar to a mini nuke blast, approximately two seconds after they are destroyed.
They can be very difficult for low level players to face head-on due to their fast speed and powerful attacks. A notable ability they have is being able to sidestep, making them difficult to hit especially with semi-automatic weapons. While their laser is fully charged they are also capable of grabbing an opponent at close range and firing their laser directly into the victim's head, which is a virtually assured death sentence.
Assaultrons' perception seems to be significantly higher than other enemies', making them difficult to sneak up on and hack without maxed-out sneak skills, corresponding armor and/or using a Stealth Boy.
Deadly especially for the unprepared, the assaultron is among the most fearsome robotic enemies in the game. While their usual unarmed attacks are already a challenge for some, the real danger from these enemies comes from their charged laser weapons. The laser cannon that doubles as its head can usually disintegrate players on contact. Making matters worse, dodging the weapon can prove difficult, since the size of the laser engulfs anything in front of it. Luckily the charge up for this weapon is long and given away by an impressive light show coming off the assaultron's face. A fast player may be able to kill it quickly or even get behind it while its firing animation is going.
The Dominator variant has a powerful attack that consists of stabbing the target's torso with two swords simultaneously and spreading its arms outwards, typically resulting in a one-hit kill.
This stronger variant of the already formidable assaultron can be counted among the nastiest dangers in the Commonwealth. The blades on its arms give its unarmed attacks considerably more bite while its head laser will still be gunning for its target. But the Dominator's true danger is its unique stealth capability, making it difficult to target, especially in VATS, before it gets into melee range. Even those in power armor should take a moment to plan how they intend to deal with this threat.
Assaultron gorgons are a modified variant of the assaultron and are created by the Rust Devils. Wielding two shishkebabs, they are the most powerful assaultron variants of all and are dangerous opponents.
One can be found with Ivey in Fort Hagen hangar and others can sometimes be seen with Rust Devil patrols.
Despite both the assaultron and the male Sole Survivor being affiliated with the military, the latter's dialogue choices with KL-E-0 would imply no prior knowledge of the assaultron model robots (although it is possible that the assaultron entered service with the US military after he had been discharged or they may have not been deployed to the front lines). This would be consistent with their absence in Fallout 3's Operation Anchorage add-on.
Assaultrons appear to be able to feel pain as evident by the quotes of the damaged assualtron found while tracking Conrad Kellogg.
It is best for low level players to avoid assaultrons as they can prove too much, due to their laser attacks, and their ability to still attack even when their legs are removed.
If the assaultron is being attacked by two enemies at once (e.g. synth relay grenade) and the NPC is targeted, if the player is too close to the head/laser while the assaultron is firing, the player still takes massive AoE damage.
Its possible that Assaultrons can express a very limited degree of human emotion. In Reunions when Dogmeat leads the Sole Survivor to the remains of a destroyed Assaultron, it simply states that it can't feel its legs, in a slight panicking matter.