Spoilers ahead for X-Men Gold #27.
This week's X-Men: Gold #27 included a surprise back-up story from writer Ed Brisson and artists Oscar Bazaldua and Erick Arciniega which flashed forward to a moment of great peril for the X-Men - the return of the Sentinels. This would be a prelude to the upcoming Extermination miniseries beginning in August from Brisson and Pepe Larraz.
Titled "Countdown to Extermination," the one-page back-up story features Kitty Pryde rallying the X-Men to fight an incoming horde of the killer mutant-hunting robots, saying "For one last time, to me my X-Men!"
As for how the Sentinels rise to this moment in which mutantkind may be at the brink of extinction again, that may lie with the villain of X-Men: Gold #27's main story from writer Marc Guggenheim and artists Geraldo Borges and Arif Prianto: Alpha, the mutant Sentinel from one of the series' earlier arcs, who has grown sentient and vowed to exterminate mutantkind - including itself.
X-Men: Gold #28 is due out on shelves May 23, while Extermination is slated to begin in August.