Quote Originally Posted by SowZ View Post
How are you going to get within a few hundred feet of the Sun's core? Also, the Sphere only sucks in matter it comes in direct contact with at a time. Even if it continues to suck in more and more of the sun, it can only destroy about 170 square feet of the sun at a time. Even if the sphere destroys its own surface area of the sun 1,000,000,000 times a second you won't even see the sun destroyed in your lifetime if you are a dwarf.
I have a feeling it's going to destroy a lot more matter than that in a second. You're essentially creating a vacuum in the center of the sun. That's the point where two opposing forces--gravity and nuclear fusion--are at their strongest. All of a sudden, nuclear fusion stops resisting, and actually starts working with gravity. Now you have the entire energy of a star set to the singular task of putting as much matter into that space as fast as possible.

However, the length of time it takes is not going to be the biggest issue for your underdark race. At some point before the sun goes out, enough mass will have disappeared that gravity will lose to nuclear explosion. The star will supernova, and the world the underdark race wants to take over will vaporize.