Going Down and sequels by Shalott (not rated, NC-17?)

[penknife.livejournal.com]

Fandom: X-MEN MOVIEVERSE
Pairing: Xavier/Magneto (with a twist)
Author on lj: astolat
Author's website: Intimations
Why this must be read:

Some Xavier/Magneto stories get you with their subtlety and the layered history between the two of them. This one gets you with the sheer perversity of a passion that's survived every terrible thing the two of them can throw at each other, twisted out of shape but never broken. So very, very, wrong, and so very, very hot. And if you're wondering why Mystique plays along, the last story in the series gives a sharp answer that rings true.

Going Down
Suspension
Second Story
Underground Levels