Drains can be considered a nuke line, granted it ends at II but it's stronger than unbuffed banish/holy and it scales with Dark Skill. Only Elemental Magic gets nuke lines, the only exception being Impact/Comet which is the only high direct damage nukes there are. The only high damage dark dmg sources I can think of are (Kaustra, Drain II, Everyone's Grudge, Eyes on Me, Dark Orb, Sanguine Blade, Lunge)