The Helm of Dominion does not contain Ner'zhul. It grants the wielder the power of the Lich King (ie: expanded consciousness and ability to control near endless undead) and has a corrupting influence, but Arthas destroyed Ner'zhul's soul.
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She would still have nothing to offer him other than service, and keeping the Forsaken in the dark would guarantee her dethroning once the Forsaken found out. If it's not malicious, why would she hide it?
Internal conflict is fine, but conflict for the sake of conflict will come off as contrived and people will disassociate.But that's the point. Without conflict, a story is worthless. The Forsaken need to look less like a hive mind, to have factions, conflicting opinions, more leaders.