Huge Maku Seed

From Zelda Wiki.org

The Huge Maku Tree is a sacred item obtained from the Maku Tree after acquiring all eight Essences of Time in Oracle of Ages, or Nature in Oracle of Seasons. It has the power to dispel evil. When Link holds the Huge Maku Seed, he is able to penetrate and walk through the otherwise impassable dark mist surrounding the entrance to Black Tower and Onox's Castle.

Although the shape of the seeds are identical, the Seasons Maku Tree gives Link a blue and pink seed, while the Ages Maku Tree offers a green and yellow seed. This could be a possible explanation as to why even though other items can be carried over from one game to another, neither Huge Maku Seed negates the darkness surrounding both Black Tower and Onox's Castle. It's also plausible that the Huge Maku Seed is somehow "used up" after it negates the darkness once.

It is the item his entire quest up to this point has been centering around, the relic he needs to face the final boss, and a treasure made of many parts recovered from different dungeons. As such, the Huge Make Seed is akin to the Triforce of Wisdom in the original Zelda, or the Fused Shadow and Mirror of Twilight in Twilight Princess.