So.. My internet was off and I decided play around with the blackbody node. IT IS AMAZING!Here is what I came up with in the hour my internet was off.
Now I am going to show you how to do the lava part which will look much better if it isn't on a cube ;)
Here is the node layout without the crazy mapping nodes
Notice the image isnt black and white aka 1 - 0 its more of a blend between 2 greys.
Black and white should work better and have more accurate color levels based on your input temperature (The multiply node) but would have a sharper falloff.
Here you can see the out put from the color ramps mostly for masking.
This one converts the noise to a black and white image (Not grey to grey like shown above)
This one defines the area that emits light (The white parts)
Here is a more advanced version of the nodes which just makes the lava swirl when its animated
Here is the blend if you want to play with it. It is CC0 so you can do what you want with it.
The first few mapping nodes in the advanced image are almost impossible for me to explain here. There is a 1 hour video dedicated to explaining it on the BlenderFoundations YouTube channel it is a must watch if you love to make procedural things in cycles.
Here are the nodes from the mushroom that weren't supposed to be seen by anyone else lol
Here are some videos including that one on I was talking about on the BlenderFoundation page