Dark champions of Necropolis.

And the archers are without bows and arrows ;)
By the way, can 25% be adjusted?!

No. And reducing the chance won't lead to anything good either, because it's their defining (characteristic) ability. If it triggers rarely, the unit loses its appeal.

There is a defense against this ability - split up the units, don't expose your strongest troops to the attack. Large units of RS with this ability are not a threat, as such damage will not differ much from a normal successful attack. The danger comes from small units of RS and single units - they can be shot down, killed with one hit, or bombarded with area-of-effect Chaos spells.