There was no rule saying the final boss had to wait until his ammunition and supplies ran out before making a move, nor was there a rule saying he had to watch the protagonists fight their way to his doorstep before taking action himself.
The most important reason Deathwing had personally taken action this time was that the two Elemental Lords had failed. Twilight's Hammer, which now occupied Wyrmrest Temple, was his last chance. If Twilight's Hammer was wiped out as well, Deathwing would truly be left unable to accomplish anything alone.
So, as the final boss, Deathwing had already prepared himself. With the exception of Chromatus, who had yet to be fully activated, Twilight's Hammer was now at its strongest. Thus, Deathwing had personally set out, working toward Azeroth's destruction.
In Quelrins City, Patrick also received Elune's warning and learned that danger was approaching.
[Be careful.] Elune's words appeared in Patrick's mind. [Deathwing has appeared. He is heading straight for Quelrins City.]
"Deathwing?" Patrick still found it somewhat hard to believe. After all, at this point, it should not yet have been time for Deathwing to appear.
[I shouldn't need to remind you. Just probe with your senses.] Elune transmitted an image of Mushroom Valley to the southwest into Patrick's mind.
Flames and fierce winds swept in with Neltharion, and the sky had already been dyed red.
"With my current abilities, could I beat Deathwing?"
[You could put up a fight, but at best, you would fight him to a draw.]
"In that case, I'd like to see what Deathwing is made of."
[You could try if you wanted, but Quelrins City would probably be destroyed in the process.]
"..."
When two mighty beings fought, the mortals below suffered. If Patrick and Neltharion fought above Quelrins City, the outcome