[ano]

You forgot one little thing about GoK. The main price is not a horror, but the fact that when you wear it, commander receives 10 madness and then +1/turn. And also some chance to get lost in time and space. My Bane Lord (ideal for this artifact, I think, together with Lycanthrop's amulet and Fenris pelt) got lost several turn ago and still didn't come back)))
When you are using soul trap, you must remember that chlll aura of the summoned ghosts may kill your commander if he doesn't have cold resistance. A perfect and cheap artifact, on the other side.
Black mirror is great, 'cause you may cast mind hunt for free, not being afraid of feeblemind. And it also dispels all illusions o the battlefield, so it's great vs Van or other glamour nations.
Twin spear is nice artifact for defense. Try the following strategy. Move a simple commander with it to the province, you expect to be attacked and script him to cast 5 times and then retreat. Chance that horror will attack him is rather small, but your (small) PD with horrors may appear an unpleasant surprise for attacker (he will summon them and retreat twice faster with boots of quickness and it is better from the point that horrors will attack simultaneously). Or even better though more risky, you may set up a big PD and script your commander to "cast spells". If the enemy army is big enough, he will still have a rather small chance of being attacked (twin spear also grants luck, so the chance is even smaller) and when your PD (you need a big value in this case) is killed, he will retreat.