Unique: The Dark Defiler

Gain 3% of Damage as Chaos Damage per Undead Minion

Dunno if this is a bug but this effect gives only hit chaos damage and doesn't effect chaos damage over time at all which means it doesnt have any effect on CONTAGION or chaos damage over time of the skill ESSENCE DRAIN.

Which makes it pretty useless since most damage from chaos spells comes as DoT and there are wands which give easily more chaos damage than this unique item.

Of course i dont know how useful it will be on other minion classes like Templar.

This is not a bug. Read how the Damage over Time works in game. They are not affected by "as extra"