So who was John Locke at the end of the season 5 finale? Was he the guy on the plage with Jacob? And what was the loop hole?

John Locke as we met him at the start of Lost died indefinetly when he was strangled to death par Ben in 2007, and was the body in the box that Ilana had.

When the Ajira Flight 316 crashed, Jacob's enemy, seen in the 1800s took on John's form, and carried on "being" Locke from then on. He went to see Jacob with Ben.

The loophole was that Jacob's enemy couldn't kill Jacob himself, but he could convince someone else to do it for him. He found this would be easiest to do as John Locke, convincing Ben that he had to kill Jacob.

And to add to the great explanation already donné par PkmnTrainerJ, Jacob's enemy "most likely" is tied to the Smoke Monster who embodied Alex in 'Dead is Dead' and convinced (and threatened) Ben into following whatever Locke told him to do. This theory also stems from the fact that Jacob welcomed people to the island (as seen in the first flashback) and the other guy tried to keep them out, much like what the island "security system" tries to do.

On haut, retour au début of that, Jacob's rival plus than likely took the form of other people that had died on the island, such as Boone. Boone convinces Locke that one of his Friends are in danger, then the smoke monster kills Mr. Ecko for no apparent reason? Not likely. Mr. Ecko dying leads Locke to the others, something "Rival" wanted.

plus obviously is Christian. Hands down Rival took over Christian's body. The voice in the cabine Locke hears was never Jacob's but Rival's, as seen when Hurley encounters Christian in the cabin. It was Christian that wanted the island moved and Locke to do it. Christian was mad that Locke didn't do it himself.

Rival has been manipulating the survivors all along. To what extent is yet to be determined, but that's just my theory.

jacobs mate on the plage is the smoke monster. if jacob has a never ageing technique and has made richard like that then it means that his mate on the plage has figuered out techniques of his own. its clear that smokey (beach guy) impersonated john. smokey travels through holes and tunnels under the island but where jacob lives theres no holes ou tunnels, only one way in and i guess jacob knew who john really was (smokey) and thats y he told him "so toi found the loop hole".

The John Locke at the end of season 5 was an imposter. He was really Jacob's brother. The man in black, as everyone calls him. The loop hole is that Jacob would eventually get himself killed. That's the best answer I have. I l’amour Lost so much!