I worked on a list on flashbacks for the Fang Gang, not just Angelus. Hope this helps. I may be missing some flashbacks from Angel Season Four, but I'm pretty sure I have everything from the rest of the Angel series and most of the relevant flashbacks from Buffy the Vampire Slayer. I threw in some the comic flashbacks in there for fun, but that list is by no means complete.
Darla A2x7, Virginia Colony 1609, Flashback 1A human Darla is dying of syphilis and is sired by The Master.
Amends B3x10, Ireland 1753, Flashback 3Liam is at a table in a corner upon which a pair of female legs dances. He is hoisting a flagon of ale and drunkenly laughing until he falls out of frame.
Prodigal A1x15, Galway 1753, Flashbacks 1, 2, and 3Liam’s father expresses his disappointment in him. Darla sets her eyes on Liam in a bar. Liam leaves his home and is disowned by his father
Becoming B2x21, Galway 1753, Flashback 1A drunken Liam runs into Darla and is sired in an alley.
Prodigal A1x15, Galway 1753, Flashbacks 4, 5, and 6Liam is buried by his family and comes back to life as a vampire later that night. Darla renames him Angelus. His first kill is a groundskeeper. Later, Angelus kills his family saving his father for last. Darla tells Angelus that what they once were carries over to what they are.
Darla A2x7, London 1760, Flashback 2Darla introduces Angelus to The Master, whom The Master does not approve of. Forced to make a decision, Darla leaves The Master for Angelus.
Quickening A3x8, York, England 1764, Flashback 1Angelus and Darla kill Daniel Holtz’s wife and children
Lullaby A3x8, York, England 1764, Flashback 1 and 2Holtz comes home to find his family dead. His oldest daughter Sarah has been turned into a vampire. The next morning, he throws her into the sunlight. Holtz's dedicates the rest of his life to vengeance.
The Trial A2x9, France 1765, Flashback 1 and 2Angelus and Darla are tracked by Holtz and hide in a stable. Darla escapes and leaves Angelus behind, forcing him to fend for himself.
Heartthrob A3x1, Marseilles 1767, Flashback 1 and 2Angelus and Darla, as well as another vampire couple, James and Elisabeth, are tracked by Holtz.
Offspring A3x7, Rome 1771, Flashback 1Angelus is captured and briefly tortured by Holtz before being rescued by Darla.
Quickening A3x8, York 1773, Flashback 2Holtz makes a deal with the demon Sahjhan to awaken in 2002 to take his revenge on Angel and Darla
Somnambulist A1x11, Europe, 1786Angelus sires a young man named Penn, who becomes an LA serial killer under the name of "The Pope"
Long Day's Journey A4x9, Europe, 1789The Beast tries to recruit Angelus to murder the Svali priestesses. Angelus refuses to help and The Beast knocks him unconscious.
Amends B3x10, Ireland 1838, Flashback 1Angelus murders a young man whom is supposed to marry the next day.
Dear Boy A2x5, London 1860, Flashback 1Darla kills a lord and a streetwalker, and, from afar, presents a girl named Drusilla as a gift for Angel.
Becoming B2x21, London 1860, Flashback 2Drusilla, in anguish for a vision of mining accident, goes to a confession booth for guidance. Angelus, having killed the priest inside, poses as the priest and begins his mental torment on Drusilla.
COMIC: Angel #25, London 1880Drusilla is forced to watch Angelus and Darla brutally murder her family.
Dear Boy A2x5, London 1860, Flashback 2After killing Drusilla’s family and driving her to madness, Darla and Angelus have sex in front of her. The decision is made to turn Drusilla into a vampire after.
Lies my Parents told me B7x17, London 1880William recites his love-themed poetry for his unwell mother, who completely adores and loves her son. She hopes for him to find a woman in his life, Williams protests that she is all he needs.
Fool for Love B5x7, London 1880, Flashback 1, Part 1At a party, William is ridiculed for his poetry for Cecily (Halfrek). Upon her rejection, William knowingly bumps into Darla, Angelus, and Drusilla
Darla A2x7, London 1880, Flashback 3On a night of feeding, Drusilla decides to find a companion and sets her eyes on William, who bumps into her, Darla, and Angelus.
COMIC: Spike: Old Times, London 1880At the party, Halfrek kills everyone who ridiculed William as wished
Fool for Love B5x7, London 1880, Flashback 1, Part 2Drusilla sires William shortly after their first encounter.
Lies my Parents told me B7x17, London 1880, multiple flashbacksWilliam introduces his sick mother to Drusilla after being sired. Wanting his mother to be healthy and eternal, William sires his mother. Unfortunately, his mother, now a demon, perverts their mother-son relationship and forces William to stake her.
Destiny A5x8, Royal London Hotel 1880, Flashback 1, 2, 3, and 4After being made, William gets to meet Angelus. Darla, however, is not present, as she rekindled her relationship with The Master. William shares with Angelus how he cares about Drusilla. In retaliation, Angelus lets William walk in on him having sex with Drusilla. Angelus explains to William that vampires take what they want. He also suggests that William should adopt a new, tougher name.
Fool for Love B5x7, Yorkshire 1880, Flashback 2William adopts the name “Spike” and is scolded by Angelus for his recklessness, putting Darla and Drusilla in danger of mobs. Spike learns about “The Slayer”
Amends B3x10, Europe 1883, Flashback 2Angelus murders a housemaid at a Christmas party.
The Girl in Question A5x20, Italy 1894Angel and Spike begin their hate for The Immortal because he slept with Darla and Drusilla simultaneously while his minions held Angel and Spike captive in "The Room of Pain". They are also not allowed to attend The Immortal's parties.
Five by Five A1x18, Borsa, Romania 1898, Flashback 1Darla presents Angelus with a Gypsy girl, whom he feeds from the femoral artery
Becoming B2x21, Rumanian Woods 1898, Flashback 3Angelus is cursed by Gypsies and regains his soul
Five by Five A1x18, Borsa, Romania 1898, Flashbacks 2, 3 and 4Darla finds Angel with a soul and chases him away. Later on, Angel finds he is unable to feed on a street woman.
Darla A2x7, Romania 1898, Flashback 4Darla begs the Gypsies to restore Angelus’ soul to no avail; they are all massacred
COMIC: Spike vs. Dracula 1898, Part 1Dracula was protecting the Gypsies, and swore revenge on Darla, Spike, and Drusilla. Spike then has a personal rivalry with Dracula for destroying a book which cost Spike 11 pounds. After kidnapping Drusilla and Darla, Spike sends a mob to Dracula’s home and rescues the women.
Darla A2x7, 1900 China, Flashback 5, Part 1Angel begs Darla back, even with his soul. However, Angel secretly only kills "bad" humans.
Fool for Love B5x7, 1900 China, Flashback 3Spike kills a Chinese Slayer during the Boxcar Rebellion and Spike and Drusilla have sex shortly afterwards. Drusilla reveals to Angel and Darla what Spike had done.
Darla A2x7, 1900 China, Flashback 5, Part 2It is revealed that Spike has killed a Slayer. The next night, a suspicious Darla forces Angel to end an infant life to prove himself, which he refuses. Angel takes the baby and leaves the group.
Orpheus A4x15, Ellis Island, New York 1902, Flashback 1Angel arrives to port from ship carrying over immigrants. To avoid human contact, Angel had hid himself throughout the entire voyage.
Orpheus A4x15, Ellis Island, Chicago, Early 1920's Flashback 2Angel saves a Pomeranian from getting hit by a car. To keep the female owner away, Angel tells her to get lost.
COMIC: Spike vs. Dracula, Los Angeles 1934, Part 2Spike and Drusilla watch a show starring Bela Lugos as Dracula and runs into the real Dracula for a second time. Dracula wants to kill Lugos for portraying him. Spike prevents Dracula from killing Lugos, fueling the rivalry. A young Ed Wood witnesses the encounter and is inspired to direct horror films.
COMIC: Spike vs. Dracula, Berlin 1943, Part 3From LA, Spike goes to Berlin to check on Darla. She had agreed to help Dracula find his bride in a camp set up by Nazi’s and is now captured. Spike agrees to help Dracula rescue to two women, but is betrayed and left behind at the camp. Spike meets two other vampires, Nostrayev and the Prince of Lies, where they would be tricked by Dracula to be lured into a trap to be experimented on by Nazis.
Why We Fight A5x13, Bottom of the Ocean, 1943Angel is forcibly recruited by the Demon Research Initiative to recover a captured German U-boat with American sailors on board. The ship also has a secret cargo of three vampires: Spike, the Prince of Lies, and Nostroyev, who were kidnapped by a Nazi team. The submarine is damaged, and Lawson, the only surviving engineer, is mortally wounded. In order to save the rest of the crew, Angel makes Lawson a vampire and forces both Lawson and Spike out of the submarine to swim to shore.
COMIC: Spike: Old Wounds, Los Angeles 1947Spike is framed for the Black Dahlia murder
Are You Now or Have You Ever Been? A2x2 Los Angeles 1952, Multiple FlashbacksAngel resides at the Hyperion where a Paranoia demon terrorizes the tenants.
The Girl in Question A5x20, Italy 1959Spike and Drusilla enjoy the time's setting. "Ciao"
COMIC: Spike vs. Dracula, Italy 1959, Part 4Spike accidentally sets Dracula free from a cursed statue when he and Drusilla visit an eccentric musician.
Lies my Parents Told Me, 1977 New York City, Central ParkA young Robin Wood witnesses his mother Nikki Wood fight Spike. After Spike leaves, Nikki tells her son that her mission as the Slayer is what matters most.
Orpheus A4x15, American Donut Shop, late 1970'sAfter witnessing a shooting at a local diner, Angel feeds on the dead victim in a moment of weakness.
Fool for Love B5x7, 1977 New York City, Flashback 4Spike kills Nikki Wood on a subway train. Spike takes her leather coat as his trophy.
Killed by Death B2x18, Los Angeles 1991, FlashbackA 10-year-old Buffy watches a demon kill her cousin Celia
Becoming B2x21, Manhattan 1996, Flashback 4Angel is found by Whistler and is directed to LA
Becoming B2x21, Los Angeles 1996, Flashbacks 5 and 6Angel witnesses Buffy’s calling and her first night as a Slayer. With Whistler’s guidance, he decides to follow Buffy to Sunnydale to help her fight evil
Fool for Love B5x7, South America 1998, Flashback 5After fleeing Sunnydale, Drusilla has a vision of Spike falling in love with Buffy and leaves Spike for a Chaos Demon.
