Ymir is millions of miles away from its sun, so far that in fact that the entire surface is covered in a thick sheet of ice.
There are, however, several oceans beneath the planets ice.
After the Daemons took control of Ymir, it was overseen by Darkshriek, with Seth or Iceflame in command while Darkshriek was away. However, when Iceflame was in charge, the Lentaa fleet arrived in the Daemoniverse through a wormhole, and invaded Ymir. They took it when the Daemon forces were at their smallest, supposedly killing Iceflame. It has since been retaken.