When they metamorphose, their eyes turn a shade of electric blue or bright gold, while their upper canine teeth lengthen to become pointed fangs.
They can also die from a broken neck, blood loss, nightshade poisoning, dismemberment, mutilation, and other extreme physical trauma, especially if their head or heart is destroyed or removed.
As explained by Selene in the first Underworld film, a bite from both a Lycan and a Vampire is lethal, but Michael's bloodline allows him to survive and metamorphose into a hybrid of both species.
Viktor viewed the child as an abomination and was acting to protect the Vampire line's purity, although Sonja's death haunted him for the rest of his life.
In order to combat the growing Werewolf threat, Markus approached Viktor, a dying warlord, to utilize his military genius and army (who became the Death Dealers) in exchange for immortality.
She was elevated to the rank of an Elder, both in order to help control the chaos created by Markus's brother William (the first werewolf; William was unable to return to human form after his metamorphosis and his bite caused even the dead to metamorphose into werewolves) and to perform the role as a tie-breaker between her fellow Elders Markus and Viktor if there was ever a deadlock in the ruling of the coven.
Before her pregnancy started to show, Amelia announced her intention to visit old friends in the Var Dohr coven in Scandinavia and appointed Thomas as her sole escort.
To keep up appearances, Thomas put it out that he'd had a low-born Vampiress for a consort who had just died during childbirth, and Amelia returned to the New World Coven with no one wiser to the truth.
It would seem that Amelia, despite siding with Viktor against Markus over William's sentence, didn't completely trust him, keeping the existence of her son a secret.
Whether Amelia ever suspected anything about this is unknown, but it would give credence to her later efforts to keep the existence and birth of her son from Viktor and his followers (and, indeed, the Vampire Nation at large).
In his quest for power, Kraven enters into a secret alliance with Lucian in order to overthrow the Elders, helping to set up an ambush for the Lady Amelia.
Tanis is a Vampire of considerable age (estimated at over 1000 years old at the time of his death), he was at one point the official scribe and Historian of the Old World Coven.
He proved to be very skilled in taming Lycan slaves with silver-edged whips, and he even used them on Lucian at Viktor's request prior to Sonja's execution.
Soren is one of the Vampires involved in Viktor's assault on the Lycan base in the first film, and he ultimately faces Raze in single combat.
David is able to use his sunlight immunity to escape Semira's clutches and learns from the Nordic Coven that he is the son of the Vampire Elder Amelia and thus her rightful heir.
The Death Dealers and Vampire Council side with David and he leads a futile defense against a massive Lycan attack before a revived and enhanced Selene arrives with the Nordic Coven to help.
His arrogance made him fail to realize the necessity of Selene's presence or what she has metamorphosed into until she used her blood to resurrect David after he died from the critical injuries he received from Lycans.
In Underworld: Blood Wars, Thomas is approached by Semira to petition the Vampire Council to grant Selene clemency in exchange for her help in training their Death Dealers.
Lycans display a number of greatly enhanced physical attributes in either form (superhuman strength, speed, reflexes, healing, etc.)
They can also be killed or permanently damaged by extreme physical trauma as normal humans are, such as mutilation, a broken neck, or dismemberment, especially if their head or heart is destroyed or removed.
As explained by Selene in the first Underworld film, though, a bite from both a Lycan and a Vampire is lethal, but Michael's heritage allows him to survive and metamorphose into a hybrid of both species.
In order to combat the growing Lycan threat, Markus approaches Viktor, a dying warlord, to utilize his military genius and army (who become the Death Dealers) in exchange for immortality.
He metamorphoses into a larger, fearsome, and powerful Lycan, has the ability to rapidly heal, is seemingly immune to silver, and is sometimes called a "Super-Lycan".
The threat posed by Marius is so great that the Vampire Council agrees to pardon Selene if she trains their Death Dealers to fight him, but she is betrayed by Semira and Varga and sent on the run.
Years later, with his wife Helena, he fathered three children, two of whom inherited the immortal strain in its active form: twin brothers Markus and William.
His blood enhances Selene's powers, making her equal to the Hybrid Markus in strength and giving her an immunity to sunlight, ultimately allowing her to defeat him.
Being a hybrid, Michael can regenerate cells and muscle tissues as well as internal organs even when he is dead, as long as his body stays intact: such as when Markus killed him by impaling him through the chest.
After an attack on a vampire coven that ends with her daughter being kidnapped, Selene confronts Detective Sebastian who admits he suspected the Lycans are not extinct.
He then shows her reports a friend of his sent him shortly before dying of over 200 suspected Lycans being tested and all coming back negative, leading Detective Sebastian to think someone might be protecting them and letting them rebuild.
Selene realizes that Antigen is run by Lycans and Detective Sebastian offers her his help in rescuing her daughter, explaining that his wife had metamorphosed into a vampiress and committed suicide to protect him.