Slain means killed, especially in battle. An old word, kept for weight.
It is the past participle of slay, and it survives almost entirely in formal, poetic and religious writing, where the slain means those killed in a war. Journalism occasionally reaches for it in headlines because it is short. American news uses it more readily than British. The verb slay has had an entirely separate modern life in informal speech, where slaying it means performing brilliantly, which nobody would connect to the battlefield.
1adjectiveslainKilled, especially in battle.
“A memorial lists the slain of both wars.”