Ysmault was a planet in the sector now known as the Forgotten Zone.
Millenia ago, the Manhunters arrived on Ysmault and devastated the planet, murdering many and causing mass destruction. What was left of Ysmault was attached to Shard, the mobile base of the Red Lantern Corps.
After Atrocitus was defeated on Oa, a peace treaty was agreed upon. At the start of the peace summit, Shard returned to Ysmault. However, the Anti-Monitor's pulse reached the planet and activated a malfunctioning Manhunter. The Interceptor crew came ahead of schedule to deal with the threat but were found out by Zilius Zox. The Anti-Monitor and his army of Manhunters attempted to invade Ysmault but were met with resistance by the Red Lantern Corps' fleet of warships. The planet was spared when Aya deactivated the Anti-Monitor, fused to its shell, and took the Manhunters with her to parts unknown. 
Cleric Loran stated that he was unsure on whether the stain glass window on Shard depicted Ysmault or another planet within the Forgotten Zone. Though, the events, and the details along with them, he spoke of are true.