Burls are a formation in the wood that is similar to a warty growth in the skin ... The grain is not uniform or straight, curling in and around itself, many times trapping bark and sapwood within the folds of the burl itself as it's formed and continues to grow over the life of the tree.
This "wild grain" pattern is unique to each burl ... literally, the "fingerprints of god", as some people put it. Due to the fact that the grain is not unified and can contain inclusions of bark and sapwood, it can be very weak and brittle, especially as those elements also dry at different rates, which will warp the burl. Stabilization is the best way to go to avoid a disappointment, when working with such an unstable and delicate material.
That having been said .... You said that it's "heavy" ... get a moisture meter and check the moisture content. If it's higher than 12%, don't mess with it ... let it continue to try naturally (air dry) or kiln dry it if you want to take the chance of large cracks.
Once it's dry enough to work down into reasonable blank sizes, snap a few pictures and get back to us with them for more advice!
I would avoid slicing the burl up at this time, as the cracking WILL happen and in unpredictable places and patterns due to the interior stresses built up as the wood dries out.
Now ... for already-dried wood (moisture content between 5% and 12% ... if lower than 5%, let it acclimate to your shop's moisture content till it stabilizes around 10 - 12% or so). I am sure we would all love to see some pictures of the burl alongside a ruler of some sort for measurement ... top, bottom, sides, and we could give you some idea of how to proceed.