Thanatos VII was the inhabited seventh planet in its star system.

In 2370, a plasma conduit was manufactured on this planet using a new interphasic fusion process, which was later installed on the USS Enterprise-D at Starbase 84. However, this new process had attracted interphasic organisms, which later infested the ship's crew and made the Enterprise's warp core malfunction. (TNG: "Phantasms")

This planet was only mentioned in dialogue.
According to the script, the pronunciation for Thanatos was "THAN-uh-toes". [1]
According to Star Trek: Star Charts ("United Federation of Planets I"), by 2378, Thanatos VII had become a Federation member.
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