The Crimson Heart was a planet upon which the Eleventh Doctor was trapped by the Claw Shansheeth.
Astronomical Data
The Crimson Heart was, as its name suggested, red, with everything on the planet having a red tint. It had at least two moons. It was located 10,000 light-years away from Earth.
One specific location on Crimson Heart was the "Wastelands", which was barren and littered with the remains of a war. (SJA: Death of the Doctor)
History
In 2010, the Eleventh Doctor was investigating the battlefield of the wastelands of the Crimson Heart when he was attacked by the Shansheeth. They were able to steal his TARDIS and stranded him on the planet. He was able to use the wreckage to create a device that allowed him to swap places with Clyde Langer, who had artron energy and was on Earth. Once there, he brought Sarah Jane and Jo Jones to Crimson Heart, allowing him to use Sarah Jane's sonic lipstick and Jo's juice to improve the device. Thus fixed, they were able to all return to Earth. However Santiago Jones, Rani and Clyde were trapped in the extremely hot vents and The Docter then decided it wasn't a good idea leaving Clyde in the same place after all (SJA: Death of the Doctor)