people who have been slain (as in battle)
"The general surveyed the battlefield with a grim expression, acknowledging that thousands of his men had been slain during the fierce assault."
Those who have been killed.
"The ancient legends speak of a thousand slain warriors buried beneath the castle walls."
past participle of slay
"The ancient warrior was slain by a sudden arrow during the battle."
killed; `slain' is formal or literary as in "slain warriors"
"a picture of St. George and the slain dragon"