The Uncanny X-Men
The Avengers and X-Man arrive at the destroyed X-Mansion seeking survivors. The teams emerge and discover Onslaught appears to combine the psionic energies of both Professor X and Magneto.
Collects The Uncanny X-Men #287-337, X-Men #53-57, Onslaught: X-Men #1, Excalibur #100, Wolverine #104, Cable #35, Onslaught: Marvel #1, Onslaught #1-6, Onslaught: Epilogue #1, X-Men / Avengers: Onslaught Omnibus. The full Onslaught saga -- Professor Xs dark side manifests as a world-ending threat.
Collects X-Men #53-54, The Uncanny X-Men #334-335, Onslaught: X-Men #1, Cable #34, X-Men: Road to Onslaught #1. The Onslaught saga begins as Xavier dark side emerges to threaten everything.
Collects X-Man #15-19 and material from 14 other series. The complete Onslaught saga - Xavier dark psyche merges with Magneto rage.
Collects X-Men #55, The Uncanny X-Men #335. The final Onslaught confrontation -- X-Men face Xavier's dark psychic entity.
Collects X-Man #15-17, X-Men Unlimited #11, Cable #32-33, X-Men #53-54, X-Force #55, The Uncanny X-Men #333-335, Onslaught: X-Men #1, X-Men / Avengers: Onslaught Omnibus. The core Onslaught crossover -- Xaviers dark psyche threatens to destroy the Marvel Universe.