Achilles will convince Sigma to join him in searching the sewers, once he hears rumor of the infected who may be living down there. Ultimately his goal is to capture the infected so that they can be properly quarantined and accounted for. He will arm himself with his sword, which is at least somewhat more subtle than his spear, and use his snazzy new defensive magic on Sigma. It is also worth noting that Sigma's weird foresight (I'm calling it dumpster fire vision; Nemo can correct me if need be) may give him some idea of where to find people so that he can reach them before a terribly calamitous incident occurs. Possibly.