(and 1 Across) Article to include on barbarian terror wasting a number under 17 at 18 (5,7)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

THREE [ HUNDRED ] (numbers of Spartans under the command of LEONIDAS [entry at 17 down]) towards the end of the battle of THERMOPYLAE [entry at 18 across])