The Rewards taken from the german wiki:

If you give Megalon one Seal you get the recipe for a demon Armor (+ 15 Endurance (? the energy you use for Specialmoves), - 10 Hitpoints)
For 2 to 4 Seals you get increasing Amounts of Money
For the last Seal you get a ring of magical Power (Magic Energie (?) +10; Magic Resistance +5, Kulminatio-Spell +5).

Edit: The Rewards stack, so if you give Megalon all five seals you get the recipe, the money and the ring.