There are 2 contradictory accounts:
According to "The Forgotten" (comic story), the Moment was built by the Eighth Doctor as a modified version of the De-mat Gun, using the Great Key of Rassilon. "Don't Step on the Grass" (comic story) adds that the Eighth Doctor activated the Moment with the Great Key & doing so was what time locked the Last Great Time War.
The Moment, seen for the first time on TV in "The Day of the Doctor", was an extremely powerful Time Lord weapon, nicknamed "the Galaxy Eater". The weapon itself was so advanced that it had developed a conscience of its own & would stand in judgement of its user. It was the weapon that the "War Doctor" intended to use to "destroy Daleks and Time Lords alike", in order to end the Last Great Time War. In order to facilitate interaction with the War Doctor, the weapon used the form of Rose Tyler in her Bad Wolf mode as its interface, having mistaken Rose/Bad Wolf for someone from the Doctor's past, when in fact she was from his future.
Unlike the account in the comic stories, the TV account has not yet (start of Series 9) revealed how or by whom the time lock was imposed on the Last Great Time War. It has only stated (repeatedly) that the events of the war are time locked.