"Nibel's source of warmth, all its glory has burned. Ori, if we enter this place we might never return..."Element of Warmth within its core, Mount Horu slowly builds towards an eruption throughout the course of the game and nearly results in the complete destruction of Nibel.
Mount Horu is inaccessible until Ori obtains the Sunstone from atop Sorrow Pass. In the original Ori and the Blind Forest, it was also a point of no return: once Ori stepped inside, they could not exit and continue to explore the rest of the forest. This has been altered in the Definitive Edition, allowing players to leave Horu at any time.
Mount Horu's inner chambers are overrun with lava, the ferocity of the volcano unable to be controlled after the Element of Warmth lost its power. Large sections of the mountain are cut off by streams of molten rock, which are fatal should Ori touch them. In order to progress, Ori must ascend to the very peak of Mount Horu and work their way down, solving the puzzles of each chamber in order to restrict the lava flow and open up new areas. All of the player's skills are necessary to prevail.
- Warning! Spoilers ahead.
After finally making their way into Horu's core, Ori and Sein discover the Element of Warmth and finally rekindle it. However, before they can celebrate their victory, Kuro arrives and attempts to hunt them down. As Horu's fires begin to leak out into the surrounding wilderness, Ori flees from the rapidly pursuing Kuro and dives into the burning trees. Sein notices the Spirit Tree not far from their location and begs Ori to get up so that they can complete their task. As the pair are about to use Kuro's Feather to reach the Tree and reunite Sein with its former home, Kuro seizes them both from the sky and severely injures Ori.
Naru, having been revived thanks to Gumo's kindness, eventually finds an unconscious Ori and cradles him. Kuro watches, realising that by hurting Ori she has become the very monster she saw the Spirit Tree as for killing her own children. She resolves to set things right and takes Sein back to the Spirit Tree. As the ancient being gains back his power, Kuro is lost in the light and a flood of calming rain soothes the fire of Horu's destruction, saving the forest.
- In the original version of Ori and the Blind Forest, completing Mount Horu would lock out that particular save file, meaning that any secrets or exploration not yet completed had to be done before taking on the mountain. In the Definitive Edition, this has been fixed to allow players to reload their save just before entering Horu after it has been cleared.
- The line, "Ori, if we enter this place we might never return..." has been changed in the Definitive Edition to read, "Let's be careful, many of those that entered this place have never returned."