The Mother Grub is a monstrous being who lives underground on the planet Alternia. She gives birth to all Trolls from a mixture of the genetic material of all adult trolls, delivered to her by Imperial Drones from filial pails. She combines all the genetic material into one diabolical incestuous slurry and lays hundreds of thousands of eggs at once.

Only the skull of an ancient mother grub, slain thousands of Solar Sweeps ago, has been seen. However, the skull bears an uncanny resemblance to Kanaya's Lusus; this is because her lusus is a Virgin Mother Grub who declined to brood and instead become a lusus naturae. Other than an extremely shortened lifespan, it is unknown what other biological differences result from this distinction.

Kanaya acquires the Matriorb from the body of her deceased lusus. It can be used to hatch a new Mother Grub. It was later destroyed by Eridan Ampora.