Example follows ...
A unit of idrian skirmisher make their intercept move. After that, the overlords made -2 for command checks cumulative so the unit has -6 on command checks.
Denegra then casts Dark Seduction on the idrians.
Next round: The wither shadow combine upkeeps one spel, dark seduction, so the complete Idrian Skirmishers (all 12) play for Cryx.