In the series
In Proven Guilty, it is stated that during the time of King Arthur he was custodian of the sword Amoracchius, becoming the first wizard of the White Council to have the honor and one of only two in history.
No one is sure how Merlin made out in the end, but from his journals, he wasn't the kind to go in his sleep.
He is believed to have saved most of the critical texts from the Library of Alexandria as it burned down, and helped to create the Catholic Church to further preserve knowledge during the European Dark Age. However, many stories told about the original Merlin are believed to be apocryphal.
He created the magical prison Demonreach by building it once, at five different times, all at the same time, adding a failsafe, a fire that would kill the things imprisoned there rather than release them, whose name, according to Donar Vadderung, is "banefire".