Evening, despite its parallel with LTT as the Morning, dies not seem like a concept directly related to the DO. The DO is darkness, true, and Death, but evening is a separate concept to darkness really. To be honest, my money is still on it being Moridin, but Mashadar is the entity with the closest link to evening, existing only then in Shadar Logoth. And Fain is built up as a power to rival SuperRand and Moridin, and it seems likely he will be present at Shayol Ghul. Both he and Moridin have spilt Rand's blood before, wounded him permanently, Rand hates them both and they hate each other too. What if he does an antiGollum and actually nearly ruins Rand, or maybe Rand and Moridin, before other Lightsiders save the day? Tomorrow I will think on this theory further