Necklace | Gurren Lagann | Core Drill

The Core Drill is the key that’s used to turn on Lagann. A small, key-like drill that Simon finds during one of his digs and is subsequently almost always seen in his possession or in the keyhole powering Lagann. The Core Drill is shown to power Lagann. The Core Drill can also sometimes be seen glowing from within with Spiral Power. (from Fandom Gurren Lagann Wiki)

Gurren Lagann, known in Japan as Tengen Toppa Gurren Lagann (Japanese: 天元突破グレンラガン), is a Japanese mecha anime television series. Gurren Lagann takes place in a fictional future where the Spiral King, Lordgenome, rules Earth and forces mankind to live in isolated subterranean villages. The plot focuses on two teenagers, Simon and Kamina, who live in a subterranean village and wish to go to the surface. Using a mecha known as Lagann, they reach the surface and start fighting alongside other humans against Lordgenome's forces before fighting the forces of their true enemy. (from Wikipedia)