It is belived that Lord Muruga teached the siginificance of 'pranava' to Lord shiva as once shiva had to forget 'pranava mantra'.This was bcoz shiva was cursed by a Rakshas to forget the pranava.Lord Swaminatha (Murugan) made shiva realize 'pranava' mantra again-thats why he is called 'gurunadha'
