A close inspection of the basidium reveals a number of indentations that might have once held something. The alien mass of fungus presents something of a puzzle. It's far from healthy, but you have little clue as to what function it might normally perform.
Perhaps someone more familiar with the denizens of Zangarmarsh would be able to tell you more about the basidium.
It looks like the basidium of a bog giant, but I haven't seen one in this condition before. I'd heard that the giants in the Dead Mire had begun to wither, but it's far more advanced than I expected.
The Dead Mire was once a lake like the others in the marsh, and it served as the spawning ground for the giants. With the waters receding, it seems the changes are preventing the giants from sporing. If this continues, the giants may disappear entirely.