Holy cr*p pantheon is totally useless late game.
I mean I was fully built before anyone and still found myself doing 1/4 of an adc's hp with my entire burst... running tls too. Wtf? At that rate I would kill them in like 30 seconds... is pantheon totally worthless late game?
Build:

is one of the strongest if not the strongest laner in the game early, so it makes sense that he falls off hard later on.