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SIL was driven by an overwhelming desire to mate. After succeeding to do so, she bore a child in an amazingly short period of time. Soon after the birth, both SIL and her child were destroyed. Unfortunately, the alien DNA infected a rodent and survived. What became of this alien is unknown.
It was discovered that the second SIL, Eve, had a psychic connection with Patrick. By exploiting this, a team was able to track down Patrick and his children. Eve and Patrick succeeded in mating before Patrick killed her. He was then destroyed using human blood with genetic defects (sickle cell anemia).
Once again, not all of the aliens were destroyed, and one of Patrick's children survived. Also, the child of the union between Eve and Patrick ripped its way from the corpse of its mother. The fates of these beings are unknown.
Each cell of the alien's body contains its entire genetic code, which can be injected into the cells of other beings like a super virus. It can replicate at an amazing speed, and it can take over a human-sized host in a few minutes. Once in a host, a SIL has a variety of forms. Its natural form is a writhing mass of tentacles. It can also retain the appearance of its host; however, to a degree, the alien can alter this form at will. This altered state has a thick carapace-like skin and long tentacles. It still retains the basic shape and residual look of its host, but it is utterly alien. Also, a "male" SIL is much larger in this form than a "female".
Infection of a host enables an alien to breed with members of the host's species. Driven by an instinct to breed, these beings will continuously pursue this endeavor. The alien can sense genetic defects in beings and will not mate with them. It has been proven that SIL DNA cannot adapt to flawed DNA, and it will die if exposed to it. A SIL cannot change hosts, because it bonds genetically immediately upon entering a host creature.
No pureblood alien has yet been encountered. The ones on Earth have acted solely on instinct, not some grand racial scheme to destroy the Earth. They live to breed and survive and will destroy anything that tries to stop them. It is not known if there is a SIL society on some uncharted planet or if they merely act as a virus preying on everything they encounter.
In natural form, this alien race cannot breed; however, it can grow at an amazing speed. In Species, one grows from the size of a single cell to the size of a small room in minutes. Its growth limits are unknown, because it was incinerated soon after. This species is required to bond with a host and use it to breed.
A SIL is a true symbiote, and most of its advantages come from its ability to bond with any known animal. A SIL cannot use a plant as a host. Once a host is taken, the SIL acquires the instincts and thought processes of the being possessed as well as retaining its own. It can still be mildly influenced by things that normally influence the host, but the SIL is in control, so this is a dangerous endeavor.
If two SILs were to breed (requiring that they possess hosts fertile with one another), the resulting offspring would be a pureblood. Such a creature has never been encountered by man. A pureblood has no host, but it resembles the hybrid form of its parents. This is the only form this creature has. A pureblood is stronger and faster than its parents, and it is fertile, but only with other purebloods. Note that purebloods do not have to mate with other purebloods of the same host species.
Climbing: In hybrid form, a SIL has greatly enhanced climbing abilities. The SIL in Species climbed and jumped with astonishing agility. The ability to climb could be greatly affected by the type of host the SIL has acquired.
Enhanced Abilities: The alien species greatly enhances the physical abilities of its host. Add 2 points to Strength, 1 point to Dexterity, and 1 point to Stamina. The host retains all memories and knowledge, but it will eventually lose control to the alien being. It takes one day per point of Intelligence before the alien asserts itself completely. Meanwhile, the host slowly begins to realize that things are not right. When threatened or presented with an opportunity to mate, the host blacks out and the alien takes control and subsequently the host has no memory of what occurs while the SIL is in control.
A pureblood takes most of its physical attributes from its parents. However, it is larger and more powerful than they, so add 1 point to the Strength and Stamina of the father SIL for the attributes of the pureblood child.
Regeneration: The SIL species' extraordinary regenerative capabilities put even the Garou to shame. The films show the SILs being riddled with bullets yet jumping back to their feet within seconds. A SIL has as many Health Levels as its host and can regenerate damage at 2 Health Levels per turn. All damage can be regenerated, even if reduced to 0 Health unless the damage is aggravated. Even lost limbs can be regenerated in record time. SILs are vulnerable to fire, acid, supernatural attacks, and particularly so to genetic defects in DNA. Such used against them causes aggravated damage. For example, in Species II a SIL is killed when pierced by a pitchfork tipped with blood containing defective DNA.
Tentacles: A SIL can utilize its tentacles in any form. For a pure SIL with no host, tentacles are the only means of movement and attack. In human host form, tentacles generally sprout from the back, the mouth, or the chest. When in hybrid form, the tentacles are on the back of the head and can sprout from the mouth, nipples, back, and possibly other locations. These do not do aggravated damage to creatures other than SILs. However, SILs are intelligent creatures and may well attack with fire or other ways dangerous to the supernatural. Their intense strength also gives them the ability to dismember their opponents.
Toxin Resistance: A SIL is vulnerable to all toxins that affect its host. However, if it survives exposure to a specific toxin, complete immunity results. It is unknown if this immunity is passed to its offspring.
| Maneuvers | Difficulty | Damage |
| Bite | 5 | Strength |
| Grapple | 6 | Strength |
| Kick | 7 | Strength + 1 |
| Punch | 6 | Strength |
| Tentacle | 6 | Strength + 1 |
Kindred: The vampires cannot become a host for the aliens. Also, the blood of the Kindred acts as a poison in the same way that defective DNA does. Thus, neither can ghouls become hosts to the SILs.
Changing Breeds: Unfortunately for the Changing Breeds, their DNA, although unique, is not flawed. Thus, most of the Bete can be hosts to a SIL. Certain Gifts may allow this infection to be stopped or slowed. The natural regenerative abilities of the Changing Breeds resist the SILs automatically. Thus, it takes twice as long for a SIL to gain total control. The Ratkin are diseased creatures, and they will not be used as hosts. Their blood can be used as an aggravated weapon against the SILs as well. Once totally dominated by a SIL, all Gnosis is lost as is the ability to step sideways. Gifts can still be used as long as they aren't powered by Gnosis. The ability to change forms is retained as well.
Mages: The Awakened are susceptible to SIL infection like any human or animal. Once infected, the ability to manipulate True Magick is lost. Although the memory of how to use magic and its effects are retained, True Magick is now forever lost to the individual. A SIL with a mage host will acquire understanding of magick, and thus, becomes very dangerous to surviving wizards.
Wraiths: Of course, a wraith cannot become a host to the aliens. However, if a wraith observes a friend or loved one become a host, it may feel compelled to warn the living of this danger. A Risen cannot become a host either.
Changelings: The fae are prime hosts for the SIL, as are most humans. However, once a Changeling becomes a host, all memory of the fae self is lost. The faerie soul is not destroyed, but it will not reawaken in this lifetime.
Mummies: A mummy cannot become a host to the SIL. Although they are still living humans, they have been altered in such a way that they are unsuitable for bonding. However, mummy blood is not toxic to the aliens.
Gypsies: Like the Changing Breeds, Gypsies have genetic differences, but they are not flawed. Thus, a Gypsy can be possessed. Bonding with a SIL destroys the Rom blood powers, but understanding of them remains.
Fomori: These poor creatures are at least safe from the host hunting SIL. They are so riddled with disease and decay that no SIL would attempt to bond with one. Their blood is not poisonous to a SIL, unless the fomor has the power Toxic Secretions.
Immortals: The Quintessence of the immortals prevents a SIL from establishing a symbiotic bond. This, however, does not make the blood of an immortal dangerous to SILs.
Djinn: The Djinn, being of a spiritual nature, cannot be bonded with.
Living Dead: The living dead have passed beyond life and therefore are not susceptible to bonding with a SIL. If bitten by a living dead, a SIL's natural immune system will destroy the contagion.
Lycanthropes: These beings are flawed to the SIL, and their blood is toxic to the aliens.
Xenomorphs: The Xenomorphs, also known as the Kainde Amedha, are the monsters from the film Alien. A SIL can bond with a Xenomorph without problem, but such a creature would be sensed by the Hive and hunted. The blood of a Xenomorph, which is highly acidic, is dangerous as an acid to the SILs, unless the SIL has a Xenomorph host.
Yautja: These are the creatures from the film Predator. The Yautja are susceptible to bonding with the SILs and would result in a creature of tremendous strength. It is probable that the Yautja, if aware of the existence of the SILs, Hunt them like they do the Kainde Amedha.
Attributes: Strength 6, Dexterity 4, Stamina 5, Charisma 4, Manipulation 3, Appearance 5, Perception 4, Intelligence 5, Wits 5
Talents: Alertness 3, Athletics 4 (Jumping), Brawl 4 (Tentacle), Dodge 2, Intimidation 2, Seduction 4 (Alluring)
Skills: Disguise 2, Drive 1, Stealth 3, Survival 3, Security 1
Knowledges: English 5, Enigmas 4 (Discovery), Investigation 2
Willpower: 7
Attributes: Strength 6, Dexterity 4, Stamina 5, Charisma 4, Manipulation 3, Appearance 5, Perception 4, Intelligence 5, Wits 5
Talents: Alertness 3, Athletics 4 (Jumping), Brawl 4 (Tentacle), Dodge 2, Intimidation 2, Seduction 4 (Alluring)
Skills: Stealth 3, Survival 3, Security 2
Knowledges: English 5, Enigmas 4 (Discovery), Investigation 2, Medicine 1, Science 1
Willpower: 6
Attributes: Strength 8, Dexterity 4, Stamina 6, Charisma 3, Manipulation 4, Appearance 5, Perception 4, Intelligence 5, Wits 5
Talents: Alertness 3, Athletics 4 (Jumping), Brawl 5 (Tentacle), Dodge 2, Intimidation 4 (Physical), Public Speaking 3, Seduction 3
Skills: Drive 2, Etiquette 4 (Military), Firearms 1, Melee 2, Pilot 5 (Spacecraft), Stealth 3, Survival 3, Security 2
Knowledges: Computer 3, English 5, Enigmas 4 (Discovery), Investigation 2, Medicine 1, Politics 3, Science 3
Willpower: 7