He lived in Camelot. Merlin had special powers. He had the power to see the future.

What did Merlin see in the crystal?

The Crystals

Merlin sees the future in the Crystal of Neahtid. The crystals of Crystal Cave contained knowledge of the past, present, and future. This knowledge could be divined through scrying.

Did Merlin complete his destiny?

It is unknown whether Merlin failed or fulfilled his destiny. Though Arthur's death would appear to have averted it, the Great Dragon told Merlin that all he had dreamt of building had come to pass.