Sentenced to Be a Hero, Vol. 4 (light novel) — Rocket Shokai

To take back Zeyllent from the Demon Blight, the penal hero unit must disguise themselves as merchants and infiltrate the city. But with the fate of humanity on the line, what will happen when their plan starts to fall apart? Meanwhile, as time runs out, a certain saint leads a band of holy knights toward the city's sturdy walls...