Question about Horrors

[onomastikon]

Can someone help me out?

I have had some success with Send Horrors, but have seen some conflicting results of horrors on the battlefield with me present. I have seen horrors attack my army as well. Does this happen all the time? I know false horrors never hurt me, but what about real ones? Are the targets of a horror random, or does it just keep on attacking until nothing is left, or are there other rules? Are certain things immune to it? Does being horror marked mean a horror will attack you out of the blue or only if one is summoned by an enemy on the battlefield you are on or if one is summoned by anyone? And if one is summoned and you are horror-marked, will it first attack you, then any other horror marked, then the enemy -- or then the nearest?
Wondering also, because some nice items give you horror mark and others let you summon them. Thanks.