The Grave of Thoughts was an infamous space-borne installation located at the Kzinti-Lyran border, near ISC territory.
During the ISC War of Pacification, the Grave of Thoughts was a trio of starbases operated by the Interstellar Concordium to indoctrinate prisoners of war. The Grave of Thoughts was the site of a major battle, called the First Battle at the Grave of Thoughts, where the Interstellar Concordium Pacification Army was handed a major defeat but at a heavy price for both sides. (TOS video game: Starfleet Command Volume II: Empires at War)
The 24th century iteration was a joint Tzenkethi-Kzinti venture for the goal of indoctrinating their prisoners, effectively making it a prison camp. In the Second Battle at the Grave of Thoughts, a Lyran count launched an attack on the starbase, asking for reinforcements when he realized that he needed to draw the enemy fleet away from the starbase. Hospital ships were dispatched to the Grave of Thoughts in order to save the prisoners before the starbase was destroyed. (RIS Bouteina: "Battle at the Grave of Thoughts")