Since the Dungeon Event went live I was farming spider for the whole day yesterday. 

I did around 100 runs and still have not dropped Treefall. 

If you compare it to any other champion that you get from any campaign there seems to be a bug with the drop rate. 

If you run a campaign on any difficulty for whole day you will get minimum 3-4 champions. 

Here Treefall does not simply drop. 

No one from my clan got Treefall to drop.

I know it's RNG, but it about 20 + people are doing runs for the whole day and no one get's Treefall as a drop, then something is wrong.

That is a champion that you need for fusion and it's not possible to get him.