The Ocarina of Time is a unique item required for casting special spells called Songs. The Ocarina of Time is required for the 22 Ocarina Songs. Four Ocarina Songs can be played using the Fairy Ocarina instead.
The Ocarina of Time is found in Kinolangdanzel Shrine, after defeating the Silver Centurion.
The Ocarina of Time, coupled with the Song of Time, is required to enter the Link Between Worlds.
Lore
As outlined in a journal found in the Freezing Caves, the Ocarina of Time was hidden in a shrine near Kakariko Village. The Dwemer who built Kinolangdanzel found the shrine and sealed it, as detailed in the Dwemer Songbook. Both books suggest that the Ocarina of Time was used as an instrument of war.
Hylian mages modified the Songs for the Ocarina of Time, but very little modification was done to the Ocarina itself.
Background and Inspiration
The model for the Ocarina of Time was originally included in a standalone mod by Buckles. It was retextured and included in version 3.0 of Relics of Hyrule with Kinolangdanzel. It was added as a centerpiece of version 3.0.
JKalenad has plans to replace the Ocarina's model and rework the texture to more closely resemble its canon appearance.
Video
The Ocarina of Time - Relics of Hyrule Video Guide