Lasar successfully negotiated with the tree. It agreed to provide Life Lightning to everyone, but at a cost, of course. It took quite some time for Lasar to successfully explain to it why they couldn't give it Life Lightning so easily. The only sources of Life Lightning were Gravis and the Freya Clan. With Gravis being more than full, giving it Life Lightning completely rested on the Freya Clan.
The only thing that they can give it were corpses of beasts. Of course, only with that, the Tree wouldn't have agreed to anything. Lasar also had to promise that, after some amount of healing, the Sect would invite someone from the Freya Clan over to give it Life Lightning.
All in all, the Lightning Sect promised to keep the tree topped-up on its storage of Life Lightning by giving it enough beasts, while someone from the Freya Clan would come in regular intervals. With this, the Lightning Sect finally got an easy way to heal their disciples.