Loonium

The Loonium is one of the functional flora added by Botania. The Loonium will, when supplied with an extreme amount of Mana, spawn monsters stronger than usual. When killed these monsters will drop items normally found in dungeon chests. However, some items, like records, Black Lotus and Overgrowth Seeds cannot be obtained this way.

Looniums are not found naturally and must be planted by a player.

FTB Infinity Evolved
In FTB Infinity Evolved, the Loonium is disabled.

Trivia
The tooltip is most likely a reference to The Legend of Zelda where the protagonist Link raises an item above his head when he finds one in a chest.