Heine has the Fiendfyre burning, but he hasn't let it burn independently until now. When Jade casts his own fire, he decides to let it go by itself. That's how it'll go in the task, he assumes.

His own Fiendfyre stays where it is for a moment. But the longer Jade's burns, the better it swallows up a roiling lion's mane and teeth and it's a phoenix sweeping around the circle's perimeter.

Heine raises his hand again and pulls. The Fiendfyre doesn't respond then.