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The moment Arthur and the group joined the battle, the tides of the battle immediately made a shift and went against the Undead Army's favor. And when Elder Zaza saw the group, he immediately knew that their reinforcements would never arrive.
The Elder stared on the battlefield for a couple of moments and watched their army get slaughtered by Arthur and the others.
Then he clenched his fist and glared at the group before turning around and entering a dark portal that teleported him out of the place. And right after he left, most of the undeads within the army disappeared all of a sudden while the rest started collapsing on the ground out of nowhere.
When the players saw that, they immediately knew that they won the battle. And to confirm their thoughts, the system made a regional announcement that the event was coming to an end with the Beast Glade Kingdom victory.