They served in the army of the Warlock Lord for two reasons.

The army of the Warlock Lord was huge and growing.

Sooner or later he would have to face the Warlock Lord.

The Warlock Lord is waiting for you, a voice whispered deep inside.

But most important of all, the Warlock Lord is real.

As with the Warlock Lord, the evil will then come into this land.

Was it saying that the Warlock Lord had come back again?

But the Warlock Lord was gone, and his creatures with him.

Perhaps the Warlock Lord had left his monsters at home.

When the Warlock Lord was before him, which would prevail?