“Personal record, Dr. Brian Virgil. This will likely be my last recording.
My requests to shut down the FEV program have repeatedly been denied. We've learned nothing useful in the last 10 years. I have to take matters into my own hands. What we're doing... it's not right. It needs to stop. If anyone should find this after... after I'm gone... know that I never wanted to hurt anyone. Anyone! Do you understand me? I'm going to make sure the whole program is shut down. If not for good, then at least for years to come. After that...I know what I'm about to do will be seen as a betrayal. Treason, he'll probably call it. So... I'm leaving. I have a plan... and if it works, I'll be somewhere safe. Somewhere not even the Coursers can find me. Everything that we've done, the lives we've taken... if there is a god, may he have mercy on us all.”— Dr. Brian Virgil Ex-Institute Biomechanics scientist turned Super Mutant
The Commonwealth strain of super mutants is unrelated to the West Coast and Vault 87 breeds, but they do share similar traits such as caging their prey, bagging organs and various body parts, and having a great sense of superiority over humans, very similar to Vault 87 mutants.
The super mutants encountered in the Commonwealth are in fact failed FEV experiments created and disposed of by The Institute. They were once innocent people, who were then kidnapped and experimented on inside the FEV lab of the Biomechanics Division under the direction of Dr. Brian Virgil. This ended when Virgil went rogue and shut down the program permanently due to the regret he had for hurting so many people over the years.
The existence of the behemoths suggests a link with the Vault 87 strain of FEV, as they are the only super mutants known to continue growing with age. However, their green-brown coloration is closer to that of Mariposa strain of FEV, specifically resembling the super mutants seen in Fallout: New Vegas. It unknown how the Institute obtained their FEV.
Their mouths are depicted as being closed much like Marcus, unlike the Vault 87 super mutants which carry a permanent snarl. Furthermore, in regards to previous appearances, specifically the Vault 87 super mutants, these super mutants have a darker tone and appear more scrawny and with less pronounced muscles.
They are roughly the same low intelligence of the Vault 87 mutants and it appears they share the ability to be some-what educated due to the existence of mutants like Fawkes and Uncle Leo from Fallout 3 and Strong in Fallout 4. Interestingly, higher rank super mutants are frequently more intelligent than their brethren, speaking more eloquently, and utilizing more advanced tactics such as cover, shifting position and taking high ground; compared to their lesser brothers who will often ignore cover and simply try and get as close to an enemy as possible, even when armed with long range weaponry.
They can randomly be encountered around the Commonwealth in small groups, usually raiding parties. Larger groups can be found infesting various buildings throughout the wastes. They utilize mutant hounds as attack dogs, these usually are encountered in groups of 1-3. These hounds are unique to Fallout 4 super mutants, instead of the revolting centaurs that have normally accompanied them in past installments.
Lower level mutants are normally armed with various pipe weapons, hunting rifles, and molotov cocktails. Higher level mutants can appear with heavier weapons such as miniguns, missile launchers, laser rifles, assault rifles, and fragmentation grenades.
Lower level mutants will often utilize wooden boards as melee weapons, with higher leveled mutants welding large sledgehammers and super sledges for melee.
The most common and weakest variety of super mutant. They carry a pipe weapon and drop minimal loot on death. They can usually be taken down with minimal effort. They can often be found amongst more powerful varieties of super mutant such as a skirmisher.
A super mutant that will run at enemies with a mini-nuke explosive device and detonate in close proximity, effectively killing both it and anything surrounding it in a fiery explosion. Shooting the bomb in the suicider's hand (targeting the right arm in V.A.T.S) will cause the mutant to explode prematurely, killing it instantly. As a result shooting the bomb from afar is the most effective way to dispatch of suiciders. The suicider can be detected before it is seen by listening for the beeping of its bomb. If the suicider is killed without detonating the bomb a mini nuke can be looted from its corpse, otherwise only nuclear material will be found on its leftovers.
Utilizing the Blitz perk, it is possible to kill the suicider in V.A.T.S. with a melee weapon before it explodes, even if it takes several hits.
Super mutant skirmishers are significantly tougher than regular super mutants, taking more damage and usually carrying more powerful weapons. Skirmishers will appear more frequently once the Sole Survivor reaches level 10.
A force to be reckoned with, super mutant masters have an extremely high hit point pool and superior damage resistance compared to their weaker brethren. They are generally equipped with heavy weapons such as the missile launcher or minigun. They can be generally found in locations with a large mutant concentration, an example being Big John's salvage.
Behemoths are the most powerful super mutants by far. However they, even when compared to other super mutants, are incredibly stupid. Despite this they are an enormous threat as they throw large boulders and use a massive fire hydrant as a bat to attack anything that comes close. A behemoth can usually be found near Sunshine Tidings co-op, and at a power station to the west of Natick Banks.
The Commonwealth variant of the behemoth is not nearly as large as the Capital Wasteland variant. It also lacks the necklace made of human skulls and car door shield. Behemoths in Fallout 4 respawn, unlike Fallout 3, where they were unique encounters. You can find a unique behemoth named Swan at Swan's Pond in Boston Common.
If the Sole Survivor manages to shoot a suicider's bomb while hidden, it will not affect the detection status, even when the suicider explodes in direct sight of other mutants. They may or may not start to investigate the body for a short period of time. This is most likely a glitch instead of a design choice.
Unlike their brethren from the Capital Wasteland or the Mojave, the Commonwealth Mutants seem to have a comparatively advanced understanding of armor, weapons and at least some minor knowledge of craftsmanship when compared to their brethren, yet they seem to trade stronger physiques for the ability to utilize/create weapon and armor mods. This is probably because of the advanced experiments in the Institute, however it is unknown if - and where - in the Commonwealth other sources of FEV might have been exposed.
Interestingly, the nukes that super mutant suiciders carry seems much larger than the standard mini nuke, appearing more similar to the Mark 28 Nuclear bombs used by Liberty Prime.
"You humans made us - now pay for your arrogance!"
"Here, human human human... I have a treat for you!"
"Brother, nooooo!" (Frequently when another mutant dies within a second's line of sight. Oddly enough this can also happen when a mutant hound dies. It is unknown wether this is an oversight or not)
"Paaaiiiiiiin!" (On injury)"
"Only one of us survives this, and it won't be you!" (Usually when they are about to die)
"A NOISE! What is it?!" (Caution status)
"Human time is done! This is the age of Super Mutant!"
"Hmph. No fight today." (When going from Caution to Hidden)