The Diamond Caves are a system of ice caverns within the Ice Kingdom where the Diamond Trial takes place. They were named after the trial's creator, Queen Diamond. The caves are also known as the Gift of Vengeance. The Diamond Caves are also where Foeslayer was trapped, until Winter set her free in Winter Turning. Winter mentions that IceWings only use their animus powers once. He also mentions that Queen Diamond had to use her animus power more than once, to create the Diamond Caves, and imprison Foeslayer.
Queen Diamond trapped Foeslayer in the Diamond Caves after Foeslayer supposedly "stole" the IceWing's last animus, Prince Arctic. She put her in shackles and enchanted them to freeze Foeslayer until she was touched by a diamond-tipped spear. The shackles would only release her if the frostbreath of a royal IceWing was breathed upon it, presumably ransom for Prince Arctic. The Diamond Trial may have started because the last time she killed Foeslayer, two dragons were with her.
Winter and Hailstorm were instructed to enter the cave and touch the ice sculpture of a dragon with the pointed tip of their diamond spears. They wandered the caves together looking for the dragon sculpture for hours. Eventually, they stepped into a room that held many frozen IceWings,some having surprised and horrified expressions. As they explored around this storage,dreamlike reality they noticed a large frozen NightWing in the midst of the frozen corpses . Hailstorm instantly lunged towards her and tapped her with the spear, unfreezing the NightWing.
As the huge NightWing unfroze, she explained her job was to try to kill them. As they fought her,it was revealed that the dragon they were fighting was none other than Foeslayer herself, enchanted by Queen Diamond to be frozen,only aging when she was summoned by the Icewings challengers.
Once Hailstorm killed her she instantly froze back into ice. Then, Winter realized one of them was supposed to kill the other and the living one would survive to tell the tale and gain glory,ascending into the First Circle. However, Hailstorm refused to kill Winter. Winter promised to leave the kingdom forever, and released Foeslayer,recognising the chain links could only be freed by the frostbreath of a Icewing. He freed her, presumably preventing the trial from ever happening again. Later, after Foeslayer had been released from the trials, she disappeared into the wilds of Pyrrhia.
The Diamond Caves are series of caves and caverns made mostly of ice, filled with stalactites and stalagmites like any other cavern, located at where Pyrrhia would breathe its frostbreath. Near Foeslayer's cave, there is an uncountable amount of frozen IceWing statues, who were previous victims of the Diamond Trial (which failed). An underground river runs near the entrance to the cave, into a dark chasm.