When the PCs arrive to the Temple of Kirill it is all swampy and quiet. They are ambushed by a group of undead swimming creatures and a big wolf. After the PCs dispose of them they find the only survivor left who updates them on what happened. The temple has a gate to the earth elemental plane. Three earth stones keep the gate open, two of them in this plane and the third one in the earth elemental plane. The dark elf who attacked the temple chained two magma elementals to the two rooms holding the earth stones in this plane. After the characters free the elementals without spilling any blood they head to the earth elemental plane. There they find a deadly injured stone elemental guard and the cavern with the third heart in total chaos. Once inside they have to battle magma creatures amidst giant roots on fire, surrounded by lava and with imminent danger of choking due to the lack of oxygen.
With almost all their energy gone and Dave one breath away from dying the PCs defeat them, repair the stone heart and heal the stone elemental who goes back with them to their world.