"The blood shall be a sign for you on the houses where you live; and when I see the blood I will pass over you, and no plague will befall you to destroy you when I strike the land..."
The rocky cavern known as Frostshard Prison was first given shape by a slow moving glacier as it flowed across the empty and snow-shrouded land now known as Kislev. First discovered by the early Ungol chieftains, its presence was largely unknown to the rulers of the emerging Gospodarin people until rediscovered by the Khan-queen Miska in the early years of her reign.
The frozen earth of the cavern lent itself exceedingly well to the magical power at Miska's command, for the very ice magic that the Khan-queen used to protect her people was of Kislev itself and capricious in nature, often trapping daemonic flesh in chains of ice instead of sending them back to the void from whence they came. Thus it is that part of every Khan-queen's duty from Miska on has been to strengthen the icy grip of winter upon Frostshard Prison and the frozen creatures within.
The Frostshard Prison is a lair hidden in an area of Western Praag, in the western mountains, by the river. It is accessible through swimming along the shore. When you discover the area, you will get a Tome of Knowledge unlock.
Approximate location of the door: 29,900 x 42,000. As you swim out, you will see a red beach on your right, door can be seen almost immediately after exiting water.
There is one large fellow who appears to be a Bloodthirster of Khorne and stands frozen in the middle of arena. There are 8 doors surrounding him. Somehow the first door is activated or opened and a hero level 40 boss will come out. It may be any one of the 8 lvl 40 hero bosses found in here. After each kill, it seems eventually another door will open and the 2nd of the 8 bosses spawns. All 8 bosses have been killed, though it is unknown if the frozen monster in the middle is an additional boss. The spawn times between the bosses seems to be inconsistent, and range from less than 5 minutes to over 30 minutes.