Artifacts

[lch]

The Horror Harmonica is great for carrying around in itself, because it lets the battle take place under the "Wailing Winds" effect, which will make your enemy rout much faster.

[esd29a]

Rings of sorcery/wizardry are good if you want to cast/forge something specific but none of your mages have enough skill. They are too expensive for general battlefield use, I think. Ring of sorcery in particular can be useful to get started in death/nature/blood as it boosts these paths but does not require them in forging.

Monolith armour (and other heavy armours) also reduces battlefield movement speed (AP) of its user. Flight might be a good idea.
I would not give it to a Myrmidon. The regeneration it grants scales with the users hitpoints - around 2-3 hp/turn on a human is not worth much. And in Dominions there are many, many spells/attacks (frozen heart, various lightnings, etc) that bypass or pierce armour. A high hp and 0 enc unit would make better use of it.

In general, expensive/unique items are rarely useful on human commanders as their hitpoints are too low.