In Overlord, could the Deep Darkness Dragon Lord be mind controlled considering it has a world item?
Yes, they'd need to steal it first. That said, unless the Deep Darkness Dragon Lord has inmunity against mind ailments, he could be targeted by regular mind control spells.

He's still one of the creatures of the highest levels in the New World, probably around level 100, so you'd need a really high level caster in order to pull a high-tier mind control spell able to affect him.