That was part of an in-zone event for Halloween I wrote up once. Something triggers the familiar zombie event, but with a twist as members of the Freedom Phalanx, recognizable NPCs and lore related zombified characters show up in the zombie horde with the final battle 1-6 members of the Zombie Freedom Phalanx. The defeat of the final Zombie Big Bads frees them of the zombie curse, sends them back to zombie dimension or dead characters like Statesman are laid back to rest until the next time the zombified heroes walk the earth.
Source Inspiration: Legion of Super-Heroes #47 (Issue)