The Immortal Corps had about a hundred members, and it was almost entirely composed of undead, golems, and other such entities that didn’t qualify as ‘alive.’ In other words, it was an entire unit consisting of beings immune to Instant Death Magic.

...

As Yogiri said that, they reached the central square. Under normal circumstances, it seemed like a place where plenty of people could relax. But now, the crowds of ugly monsters filling the place meant there would be no respite here. Numerous rotting corpses and clattering skeletons wandered around. Gargoyles patrolled the sky while stone giants watched from down below. This must have been the Immortal Corps.

...

Infuriated by Yogiri’s casual attitude, his expression was the only part of him that changed. “If all you’re gonna do is make me mad, then screw talking. Just hurry up and die!”

Masayuki jumped to his feet. As if it had been a signal, the undead wandering aimlessly around the square all turned towards Yogiri as one.

“Die.”

But with one word from their target, the atmosphere of the square changed completely.

The wandering undead collapsed. The gargoyles overhead turned into stone statues and fell to the ground. The stone giants toppled over, crushing the other monsters beneath them.

In an instant, the entire Immortal Corps had been wiped out.

-Volume 1 pages 136, 227, and 229