Fireweed

Fireweed has no leaves, just endlessly branching black, spongy stems with purplish-red sap. The whole growth looks like an enormous drape of Spanish moss.

The plant poses little danger to most explorers, but fire is useless against it. Heat of any kind, magical flame included, only feeds it: for every die of fire damage it absorbs, it grows another foot. Enough heat can make it fill whole passages or caverns at a magical pace, and a steady heat source can let it choke every open space for miles in a dark, woody jungle.

It thrives in parts of the deep underworld so harsh that even fungi die there. Like surface scrub and saplings it grows in stands, and each stand is nearly always one organism, a founding plant cloned hundreds or thousands of times over.

Fireweed's partly magical nature turns heat into food and poisonous fumes into breathable air. That can make volcanic zones survivable and turn barren stretches of the underworld into fairly fertile ground. Deep dwellers value it enormously and treat it with respect, and surface folk who cut down a stand in front of them often pay for it.