The Terracotta Warriors can be summoned by sacrificing the Eye of Izanagi - one of three artifacts earned by completing the Green Meteorite Quest (the last quest; you cant go back and do the others first) at the Museum. It fills your party capacity with the Terracotta Warriors. Take care however, for the Eye of Izanagi is also the only artifact which can bestow normal sight to blind characters, when used to banish fog, just like the Evil Eye.