a joint grave for many people who were killed in the same battle, died around the same time from the same cause etc
common (mass) grave.
...Теперь позаросли бурьяном высокие холмы братских могил, придавило их дождями, позамело сыпучим снегом (Шолохов 3). And now the high mounds of the common graves were overgrown with weeds, settled by rain, and canopied in drifting snow (3a).