No sooner had the scream died away than the head of a great serpent rose out of the flames, with the diary in its mouth. It tossed the book into the air and then lunged to swallow it. Then, his eyes growing wide, Severus watched as, as if in slow motion, the snake turned it head to face him, intent on striking. Though once it did, its head collided with Magius's wall of wind, and it shook its head, opening its jaws in a silent hiss.
The remainder of the fire also started to writhe with the yellow outlines of various beasts, all of which fought with the barrier pinning them in. Almost at the exact same time the thought occurred to him, it seemed to occur to the Fiendfyre, as well. If it couldn't expand outward, it would have to go upward. The fire rose up, up, five -- ten -- fifteen feet, before the serpent shot above the top of the flames, bending its head toward him once more.
"Aguamenti torrens!" Severus shouted, pointing his wand at the snake's head. And he was glad he had added the 'torrens' at the last second, because when the water collided with the fiery serpent, more than half immediately hissed into a cloud of steam. A regular Aguamenti spell would never have worked. But it was at least forcing the snake backward, away from him.
Once he was certain the fire wouldn't try to pursue him again so soon, Severus glance back over his shoulder, with a hurried, "Harry -- help me!" He wasn't certain whether Magius would have to drop the wall of wind before he could cast the dispel magic, but if that happened to be the case, he would need all the help and water he could get.