1) If you are clanless, you can't use the special ability against monsters.
2) If you are in Bifrost map or in the War maps, you can't attack humanoid-enemies. They even get healed back for 1920 , in spite of getting damaged for 960hp
3) If you are inside any arena (colosseum included) you can't use the special ability. The game's mechanic neither lets you cast the spell on a humanoid-enemy.
4)As it happends for every priest's hammer (like lobo-lupus hammer, smite hammer,Holy animor etc..) you should be able to put the dark mace too on the offhand . But u cant.
5)Hammer's elemental damage does not work. The elemental dmg on any int -based priest hammer (Dark mace included) does not work.
Several tests were done comparing [Holy anymor+9 (123/0)] with a [smite hammer+8(122ap/90flame) + 14 int title (gone mad) for compensating the int bonus on the holy animor]
Both against mobs and against the same humanoid-enemy ..and ofcourse..under the same circumstances.
In both cases, you deal the same range of dmg. While, the smite hammer should deal much much more dmg cuz of the elemental bonus.
If u compare any other weapon - for example, a raptor+9 with no elem dmg versus a "normal" raptor+9 with elemental dmg...you will see a huuuge difference in term of dmg dealt.
https://youtu.be/VKCpVo4aIls