What was the thought behind Nasus?

His kit screams late game duelist, but he's built as a tank His trade off is suppose to be a bad early game but his passive gives lets him easily counter that Tanks are supposed to be damage soakers with low damage. Bruisers are somewhat tanky and somewhat damagey. He builds full tank and deals damage like a burst assassin.

Nothing about Nasus makes sense at all. His entire kit is putting the enemy in his E while withering them and ults to gain that advantage while Qing them. At first look with that kit he seems like just that a duelist. Why is he also incredibly tanky with great sustain?

Note: This is in no way a rage/complaint thread about Nasus. I find him easy to beat and just won a game vs him. I'm just legit curious what was nasus originally suppose to be? A tank or bruiser? If tank then why does his damage scale forever? If bruiser then why does he get so much tankiness?