I've been reading a lot of lore the past 3 days, basically nearly everything from WC1 to Cataclysm <.< but anyways, I was on the toilet and I had a revelation.
What if we don't actually kill deathwing? I mean, He is known for running away when the going gets tough and if he is this great being of immense power that can tumble the world in the Hour of Twilight with the old gods.. then how do a mere mortal kill it?.
So what if we got to a certain % and he runs off only to come back when the old gods ''whisper'' him to come back, if you know the lore you would know the assumptions that he is there greatest weapon.
So maybe we haven't seen deathwing at full capacity and maybe never will?.