A very compelling argument: take a Death Lord and give it to a level 35 hero, and – bam – not a single point of Dark Energy. Moreover, I even tried to have all 8 Necromancers, level 35, capture this Death Lord, and still, no Dark Energy points. Conclusion: the Death Lord does NOT affect the amount of Dark Energy. And if that's the case, then it's almost useless to take it in the late game, because there are plenty of strong neutral creatures around, and 5% is nonsense.