Excession - Iain M Banks (Book 5 in the Culture series)

Two and a half millennia ago in a remote corner of space, beside a trillion-year-old dying sun from a different universe, the artifact appeared. It was a perfect black-body sphere, and it did nothing. Then it disappeared.

Now it is back. Diplomat Genar-Hofoen of Special Circumstances is sent to investigate but, sidetracked by an old flame and the spoiled-brat operative Ulver Seich, and faced with the systematic depravities of a race who call themselves the A­ffront, it's anyone's guess whether he'll succeed...