Harry & Dudley flee an abusive Vernon - to Severus Snape. Severus finds a home for himself & the boys with dragons and hunt the Horcruxes from there. The dragons - especially one - become their allies. Tom R is VERY different.
— Summary on FanFiction.net
3 Slytherin Marauders is an in-progress AUHarry Potter fanfiction written by severusphoenix, found here.Petunia Dursley, Dudley Dursley, and Harry Potter leave 4 Privet Drive and - after Petunia recovers from being shot - take shelter on a dragon reserve, thanks to Severus Snape. With help from Lucius and Draco Malfoy, Rosmerta, Tom Riddle, ex-Auror Hugo Savage and a dragon named Yvane, they start hunting Voldemort's Horcruxes.
3 Slytherin Marauders provides examples of:
Abusive Parents: Vernon Dursley, like in canon. Unlike in canon, Petunia is not abusive.
The Bait: the Aurors have Lucius the "Death Eater traitor" spend a night in lockup in order to lure more Death Eaters out of the woodwork.
Bavarian Fire Drill: Draco throws a tantrum and Lucius demands the Ministry workers stop whatever they're doing and assist him so that Arthur Weasley can sneak in to find evidence that Rufus Scrimgeour and Dolores Umbridge are plotting to steal Harry away from his family.
Big "NO!": Tom when he's sorted into Gryffindor. Also, in a more dramatic example, Snape when Harry's name comes out of the Goblet of Fire.
Bitch in Sheep's Clothing: Dumbledore. He hides behind his "kindly old grandfather" persona but he dumped Harry at the Dursleys and paid Vernon to abuse him so he can later swoop in and "rescue" Harry and indoctrinate him so that he will gladly die for the greater good.
Dating Catwoman: Lampshaded with Tom and Dora, with Dora being an Auror-in-Training and Tom who the Wizarding world knows hails from a "dark family" and unknown to the wizarding world is an alternate copy of Voldemort himself.
Good Is Not Dumb: Arthur Weasley thinks up a diversion that impresses Lucius Malfoy.
Grave Marking Scene: After venting about his abandonment issues Tom gets some closure and leaves flowers on his father's grave. Later when Voldemort stumbles across the flowers, he's greatly puzzled as to who would give that S.O.B. flowers.
Groin Attack: when her ex showed up and threatened Harry, Rosmerta hit him with a hex that would "tie his penis in a knot."
Happily Adopted: Harry, Tom, and Beth by Snape and Rosemerta. And Lucius and Petunia get married so Dudley and Draco become step-brothers, and after finding her they adopt Lucius's daughter Victoria as well.
Jerkass: Mad-Eye Moody, Vernon Dursley and Sirius Black. Remus Lupin has shades of this as well.
Just Deserts: McNair learns too late why you don't meddle in the affairs of dragons...
Knight Templar: Albus Dumbledore he encouraged and paid for Vernon to abuse Harry in order to turn Harry into a weapon that would be completely loyal to him and would be okay with dying. Also Alice Longbottom and Neville has inherited these tendencies as well. And Rufus Scrimgeour.
Loophole Abuse: The Goblins inform Tom that he can't completely empty out Voldemort's Gringott's vaults... so they leave him a single Knut. Also fugitives can seek asylum at Gringott's because "Gringott's customers are allowed to count their gold for as long as they like."
Luke, You Are My Father: An exploited trope when Snape and Lucius pass off Tom as Voldemort's son. Later played straight when Lucius discovers Victoria, a little girl that bears an uncanny resemblence to him.
Miscarriage of Justice: Scrimgeour throws Snape and Lucius into Azkaban without even charging them of any crime. One would think the Ministry would no longer try to do something like that after Sirius Black was cleared.
Orphanage of Fear: Where Tom grew up. Also Tom and Harry rescue Beth and Tori from one.
Papa Wolf: If you try and hurt Harry Snape will end you if Tom doesn't do it first. Same goes for Draco and Victoria
Parental Abandonment: Tom has these issues with his muggle father because as far as he knows his father just abandoned his mother for no reason. He also realizes that because his other self killed his father years ago he'll probably never know the full story.
Pity the Kidnapper: after Voldemort possesses Lockhart, Lockhart begins driving Voldemort (more) insane.