Bad decisions, an incompetent leader.
The liches led the undead well, the vampires were smart and followed orders, it was no longer a small war with messy enemies.
The enemies this time knew how to take advantage of Viktor's lack of ability to command.
The veteran Rimv realized this, but it was difficult to communicate it to Vik, who was fighting in the heat of battle.
The liches were hard to kill, and there were so many of them, it was unrealistic for one man to try to kill them.
Of course, they were not liches like the mighty space lich...They only controlled part of the undead troops, but with their considerable numbers they served well to make his army more controllable.
Viktor did not coordinate correctly with his troops. He realized too late that it was a mistake...
He wasted time and energy, and his soldiers who were left behind were exposed.