Artifacts

[Psycho]

Horror mark is Thaumaturgy 1. It is not bad form to attack and retreat. You can attack, drop a horror or two and retreat.

But, bear in mind that horror harmonica horror marks the one carrying it, so he will be the first one to get attacked by horrors. That's why you want to bring a horror-marking mage along. It may be safe to use it on the first turn you equip it, before it horror marks the one carrying it.

And if you are not teleporting the one carrying it, it is better to put it on a regular commander and give him boots of quickness, so he'll cast two horrors per battle round. Even better to use a more durable commander like bane lord or firbolg. I've found out that a mage won't use horror harmonica after the third horror and will switch to casting spells.