The will of the plane can see everything happening in the plane both the surface and the Underdark so it probably knows where Ragnarok is. But it cannot do anything with the information it has. There's no faction for it to order around to do its bidding. At least not yet.
As for the demon god of Carnage, its main body, or at least the body it is using to attack the plane is still outside the plane. It can only rely on its spawns to serve as its source of vision and also its vague sense of life.
Ragnarok would have been better hidden from the demon god in a crowd of living things. But he ran to the end of the plane where there is no life whatsoever. His life force shone to the demon god like a torch in the dark.
The torch was hidden here by something but the demon god can still sense the heat from the torch. Ragnarok also left a trail of energy and life that ended abruptly at the base of the mountain. All these clued the demon god to where Ragnarok might be.
