The Primordial Frost Monarch had died back in the Ice Age. Only its soul lived on.
A famous saying was prevalent during the Ice Age – 'The Frost Monarch is immortal, at least, as long as there is a spec of ice in the world, she cannot die.'
The frost monarch did live up to that name. Despite her body being dead, her soul survived all this time.
Well, this was another characteristic of beasts that had reached a very high level. The Divine or mythical beasts and the Primordial Beasts lived on with only their souls. As soon as they would find a suitable body, they could get another chance at reincarnation.
Apparently, her soul wandered and got trapped in Ancient Ice Runes. When Demon Guild found it, they started working on its resurrection and controlling it.
As mentioned earlier, it is believed that Primordial Beasts have more destructive power than the Mythical Beasts, hence, what else more could the Demon Guild ask for!?