Board Thread:General Discussion/@comment-27464043-20151230192612/@comment-28278649-20180411163002

The Flame isn't what controlled the Beserkers though, it merely created them from dead Asgardians.

And, since it's fire, and Surtur is a Fire Demon, I'm willing to bet such a power is naturally occurring for him. How they managed to either steal it from him or prevent him from obtaining it, who knows.

However, the only Stone shown not to originally be solid matter was the Aether. Every other Stone was already in their 'regular' form.

It's more likely that the Soul Stone is something that has to be powered, or fed like in the comics before it can be utilized than it being the Eternal Flame.