They would gather such Undead and take them to the Dark City. The major activity of Dark City was to wander about and scour the world for Undead, teaching or guiding them, rescuing them from human territories.
Over the centuries, Dark City had assembled more than three thousand Undead and over twenty thousand members of various life species. Yet they constantly had to contend with the onslaught of the Church of Light. Of the twelve knights, only Rogge remained.
A while ago, a corner of Dark City received a secret code specially for Piero, informing him that there was news from the Resting Palace. The God of Knowledge had transformed into a bronze Juvenile Dragon and arrived in the human world.
So when Jerika told him that there was a bronze Juvenile Dragon under Lord Ange, he immediately thought of the God of Knowledge. Once he confirmed that Ange had the ability to purify the Undead bacterium, he brought everyone over immediately.