The Bustling Fungus is a returning item in Risk of Rain 2. It creates a green bubble which heals players for 4.5% health every second after the user stands still for 2 or more seconds. Both the radius and healing amount can be increased by stacking, up to a maximum of 100+% healing every second at 44 Bustling Fungi and with no effective limit on radius.
This item is extremely powerful to stack as Engineer, due to his turrets' ability to inherit his item inventory and their immobility making the healing bubble persist wherever the turret is placed, however it is almost completely useless for every other survivor. As such, this item should be saved for an Engineer on your team if you're playing multiplayer.