From Sonic Retro
Merlin was the grandfather of Merlina the Wizard in Sonic and the Black Knight. He never appears in the game, and his fate within the SatBK continuity goes conspicuously unaddressed. It seems likely that he has simply died of old age some time before the events following Sonic's summoning, considering that his grand-daughter is by now a young woman and accomplished wizard in her own right.
Years previously in the Arthurian world, Merlin created the illusion of King Arthur as a means of bringing peace and just rule to the Grand Kingdom (explained by Merlina just before Knight's Passage). However, the Lady of the Lake gave Arthur the Scabbard of Excalibur, thus making him immortal. The power corrupted Arthur, plunging the Kingdom into darkness. Indeed, Merlina describes the fallen King as "my grandfather's greatest mistake".
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Songs: "Knight of the Wind" | "Through the Fire" | "Fight the Knight" | "With Me" | "Live Life"