The high-ranking Necromancer responsible for central command was killed, which directly led to the loss of command within the Elite Skeleton Army.
Despite there being thousands in the Undead Army, it can't be possible that there's only one Necromancer as the commander. But given its formidable strength, naturally, it was at the core. Without its control, it's like a beehive losing its queen; the remaining lower-level Necromancers, no matter how hard they tried to rally the troops, would be unable to restore the already chaotic Undead Army back to combat readiness.
The vampires hurried over from nearby watchtowers, barely making it in time to clean up the battlefield.
The most challenging part had already been completed by Old Qin and his young "apprentices"; what was left was just to sniper-kill the low-level Necromancers to completely eliminate the threat of the Skeleton Army.