A planet made largely of dilithium inside the Verubin Nebula, where a Kelpien ship crashed and the child Su'Kal unknowingly caused the Burn, as revealed in Discovery's third season.
Theta Zeta lies deep within the radiation-soaked Verubin Nebula and holds a vast natural deposit of dilithium. In the Discovery episode Su'Kal, set in 3189, the crew trace the source of the Burn to a wrecked ship on its surface.
The vessel is the Kelpien science ship KSF Khi'eth, which came looking for dilithium more than a century earlier. Su'Kal was born aboard, and exposure to the planet before birth linked him to dilithium itself. When his mother died, the child's cry of grief spread through subspace and made dilithium inert across the galaxy, destroying every ship with an active warp core.
Saru, Culber and Adira enter the holographic world that raised him and, in That Hope Is You, Part 2, help him face the memory and leave. The planet's dilithium then becomes vital to the Federation's recovery.