Nightstalker is this buffy, scary looking blue vampire thing and you would think he is some kind of hard carry but nope... he is basically a support, and not a "melt your face" kind of support like Ogre Magi, but more of a control / utility one like Rubick.

On Dota 1 / early Dota 2 you could snowball him by playing like QoP of Pain but those days he is on the low end of late game power.

But given his utility is very unique with the night vision shenanigans, this give him a unique niche as opposed of being another strenght semi-carry people will never pick.