Brother Assassin, by Fred Saberhagen: Second in the "Berserker" series. A "fix-up" of 3 novelets from If and Worlds of Tomorrow - all revised: Stone Man / The Winged Helmet / Brother Berserker. The planet Sirgol had been burned off by berserkers - machines set to destroy life wherever they found it. But way down, under miles of rock, one-tenth of the population survived. For the majority, time was an endless twilight devoted to holding off the final, definitive attack by the berserkers. Time was what they needed for help to get to Sirgol. And Sirgol had special properties which made all of time open to the inhabitants - but this just put them in worse danger when the berserkers found this time-travelling phenomenon.