I tried to use that spell once when the last remants of the Altarian Kingdom were approaching a carelesly defended city of my own. I wanted to soften the opposition and charge. So I cast it...

But... what's that?

Apparently there was some kind of graphical glitch. Right? Nope!

They were there and kicked ass (I won the battle though).

Many turns later, another example of "Vertrar's Howl" awesomeness when my Deadly army is considering beating the Epic army of "Delin, the Pyre of Man's Army". So it was a good chance to howl a bit and...

By that time I have Csilla in my army, who casts Mass Curse at the beginning of the combat automatically. I have tried the battle and couple of times and in addition to having those "undead" creatures in the opposing army, as soon as that battle starts she casts her spell and all the opposition (including Delin with his 154 hit points) dies:

Savegame for your easy testing: "Epic" fight. It's just before my army reaches the Elemental Lord.

No debug.err but... *points at saved game*