Using clips from Supernatural and Angel, a story has been constructed in which Evil!Cordelia teams up with Lucifer to bring about Armageddon. In the meantime, before John Winchester dies, his big secret to Dean is that there is a secret half-brother that the boys never knew about. No, no, it isn't Adam (although Adam being changed into Adam would be interesting too - but that would be a Buffy crossover).
Not just any Wes, either, but down-in-despair Wes.
Anyway, the Winchesters have to unite to stop the apocalypse, with Lucifer knocking up Cordelia to bring forth the Anti-Christ. It's in 7 parts, so this is a multi-chapter link & the creator is: blueweasel1992.
Supernatural/Angel: My Three Sons Chapter 1: A Father's Deathbed Confession
Chapter 2: Too Late
Chapter 3: Enemy
Chapter 4: Family
Chapter 5: Fight or Flight
Chapter 6: Balance
Chapter 7: Lamentation