ETA: It's not Snarry after all, it's Harry/Draco. Mods, should I take this post down?
The fic is
The Higher Geometry by Lomnaaeren
I'm looking for a fic, possibly set post-war, where Harry and Snape and a couple of Aurors or Unspeakables are in the Dept. of Mysteries. A new kind of prison has just been developed to deal with overcrowding? In Azkaban. The criminal is meant to touch this artefact (a stone? A crystal?) and is sucked into it. Time runs differently inside the artefact, so 20 years in the prison is something like 20 seconds in the real world.
Of course Harry and Snape accidentally touch the artefact and are accidentally sucked in.
Thanks in advance.