Majora's Ocarina is the Level 3 weapon in Skull Kid'sOcarina moveset in Hyrule Warriors and Hyrule Warriors Legends. Its design is based on Majora's Mask and befitting dark power of that Mask it has a Darkness Elemental Attribute. Interestingly, its seems to alter Tatl and Tael's appearance slightly, indicating its dark power may be corrupting and/or empowering them.
Theory warning: This section contains theoretical information based on the research of one or several other users. It has not been officially verified by Nintendo and its factual accuracy is disputed.
Given its design and association with Majora's Mask its possible that Majora's Ocarina may have been created by Majora itself and or is an Ocarina imbued with its dark power.
It may also have some connection to Lunar Ocarina (Skull Kid's Level 2 weapon) that has a design based on the Moon.