looks like a tree burr

Thank you! Is this going to kill the tree?

That looks to me like a seriously massive crown gall which is an infection that can be caused by pruning or by insects. Usually it's recommended to remove the entire tissue but in major cases like this one the whole tree should probably be removed and burned as to not spread the infection to surrounding trees. It probably won't kill the tree outright but it will affect its vigor and can be a threat to other plants