Darla is born in the late 16th century in the Colony of Virginia. She is Sired by the Master of the Order of Aurelius in a monastery in 1609, while on her deathbed (she was dying from syphilis). In her human days (which lasted an unknown number of years, but I estimate her to be in her early 20's when she's turned) the woman that later becomes known as Darla is a whore. Her origin is unknown, but learning from her choice of profession, she didn't come from a higher class or a respectable or stable family.
Her human name is unknown, and it was also the name that served her for several decades after she was turned into a vampire. The name 'Darla', Anglo-Saxon derivation, means 'dear one' came into common use over a hundred years after she was born and was probably given to her by the Master, whose mate she was at the time.
In 1753, Darla discovers Liam in a bar, during one of his infamous drunken sessions and refers to him as 'magnificent'. She can feel the power about him, his glory and his potential. That very night, she allures him into a dark alley and promises him the world - she turns him into a vampire.
She and her new mate wreak havoc for 7 years through Ireland and England, until in 1760, she returns with Angelus to the Master in London, and introduces her Sire the newest member of the Order of Aurelius. After a dispute between Angelus and the Master, Darla has to choose - either she stays with the Master, stays loyal to her Sire, or leaves with the young vampire, Angelus. She sides Angelus and they depart from the Order.
For 150 years they both tour allover Europe, leaving a trail of mass murderers and bodies after them and create a worthy reputation to themselves. Together for a century and a half, Angelus and Darla aren't only 'murder partners', they're lovers. Neither one of them loves each other, it's obvious. Darla is as loyal to Angelus as she was to the Master when she walked out on him (she doesn't think twice before saving herself, for instance, and leaving Angelus to die), though the attraction and sexual rather than emotional bond between them is great. Their bond grows very strong, as much as it can between two soulless vampires, but still, when in 1898 Angelus is cursed with a soul, Darla refuses to take him back. According to her, he's not her mate anymore - he's not Angelus, hence he's no longer worthy of her companionship. Still, she does lead Spike and Dru for a slaughter allover the Gypsy camp that is responsible for the loss of her lover (which brings us to question the reliability of later-to-come Jenny Callendar, if not a person of the tribe survived that attack). Unable to rid him of his soul, Darla agrees to give Angelus a last chance, but he fails to please her. They part in China, 1900, during the Boxer Rebellion, when Darla challenges the ensouled Angelus to murder an innocent baby to prove himself to her. He can't, and grabbing the infant, runs away. This is the last time they see each other for decades to come. While Angelus travels to America, Darla returns to her place by the Master's side.
She shows up in California in 1997, in the town of Sunnydale that lies on the Hellmouth, preparing, together with the remains of the Order of Aurelius, to restore the Master back to life and to full power. That year, she meets again with Angelus - now Angel. His love for the Slayer sickens her more than the soul used to at its time, and more importantly - she's jealous. She does everything in her power to twist his feeling and feed from his most inner doubts, even frames him for attacking Buffy's mother, causing him to practically wind up on the pointy edge of Buffy's stake. And he does, but Buffy is the one who doesn't kill him. Realizing she failed, Darla lets her jealousy cloud her judgement and goes after the Slayer herself, which ends up in her death, by no other hand than Angel's. He reduces his Sire and former lover to dust to save Buffy.
Only years later, in 2000, Darla is resurrected from the very depths of Hell by Wolfram&Hart, as a human being - no longer a vampire, in Los Angeles, California. Of course, she is resurrected with the sole purpose to get to Angel - to play with his weaknesses and strength like only she can until he'd slip, to bring him to the dark side. Darla is all for it, she doesn't mind at all being basically used by the low firm if it means getting her lover back.
But things don't go as planned. Her soul, now that she's human, starts being the burden that it is, with 400 years of murder and torture on her conscience. Unable to deal with it, she cracks, and practically begs for Angel to turn her back to a vampire only to stop that torment. He won't. Instead he wants to help her, save her soul.
But they both soon learn there is no time for that to happen - Darla is dying. The disease that was killing her in her human days comes back, with her body being human again and she is, again, dying. Unless he turns her. still, Angel refuses. He can't turn her. Darla's humanity is something he can't take part in taking away from her. It's something *he* ceaselessly dreams of, he views it as a priceless gift of redemption, he can't force himself to rid her of that gift. Instead, he tries to help her and undergoes suicidal trials to get her a second chance in life, but it all turns out for nothing. After all, she's *living* her second chance. So she's doomed to die.
But she never does. Just as both she and Angel finally manage to come to terms with her sealed fate, Wolfram&Hart bring Drusilla to turn her, seeing Angel has proven to be useless in that area.
Vampire again, Darla is up to her old tricks and this time Angel is out for the kill. He knows there's no chance for salvation for her this time, and he knows, just as she does, that the next time they see each other, it'll be a fight to the death.
In the heat of despair, Angel gives Darla what she has been after forever - the chance to rid him of his soul - he has sex with her. In the morning after though, his soul is still intact and Darla is furious (which is actually, surprising, because she did sleep with him after he gained his soul by the Gypsies and he didn't lose it, so why should she be expecting something else?). seeing he's been given a second chance, Angel returns to his senses and makes it clear to her - she will get dressed and leave. If he ever sees her again, he'll kill her.
She leaves, having nothing left to look for in Los Angeles anyway, especially after the humiliation of not having lifted Angel's soul, which Buffy *did*. But Darla soon finds out she has kept something from that night of passion - she's pregnant. And she's not only pregnant, but she's pregnant with a child she can't get rid of, no matter how hard she tries and who she turns to for help. She visits every powerful Shaman or wizard she can think of, but all tell her the exact same thing - the baby inside her, that no one even knows how it could survive for that long in a dead body, cannot be killed. The pregnancy can't be terminated. There's nothing anyone can do.
Time to go visit daddy.