Patience is hard

At first, Zain didn't know what the Dark Zombies were, but having spent time with the Reborn organisation and with Sarah, he had quickly learned about the group. Before joining the Reborn group, Zain had a run-in with them before.

A strange old man seemed to be controlling the Chained Zombies that, at the time, Zain was unable to defeat. After that event, he had travelled solo, trying to do everything he could to grow stronger, just in case he ran into them again.

The thing was, he never did run into them again, yet he had heard of others encountering the Dark Zombies. One of the rules of the Reborn group was not to engage with those who called themselves Dark Zombies and to run away.

As for why, Sarah never really explained. Other than the few details she had told him personally, he had his own guesses… that they were a group of strong Reborn Zombies, much stronger than the Reborn group. Even Sarah knew this, which is why she told everyone to run away.