it says the "effect lasts until the end of the game"...and then "this ability can only be used once, ..."
i take it the "effect" is the "whenever a until enters your territory, put an augment counter on it"...if i'm correct so far, what's the purpose of the "this ability can only be used once" text? since it remains in effect why would i ever need to use/activate it again?
The intention from that wording is that the Aegis of Strength hero ability will only ever be used once in a game. The ability doesn't ready itself, and a player couldn't use it again to put a second Aegis into play.
The Aegis itself lasts for the rest of the game though, and continues to have an effect on the game.