Level Drain. If Halsin explained that he got captured because an associate of Minthara's, who returned to Moonrise after the attack on Waukeen's Rest, had used a medallion or magic item of some kind that drains people's levels, permanently, that would do it. Halsin was once a level 15 or something druid. He went with the adventurers to check out the cult at the temple of Selune. They were running. The Drow used the magic item and drained Halsin's levels. Goblins swarmed him. He fought hard, but each round the Drow drained another level until he used up the magic item's 10 uses. Now, Halsin is only level 5. He has significantly lost his power because of the wicked Drow's amulet. They could even call it the Vortex of the Absolute, or something crazy like that. Halsin, therefore, could be explained as being so weak because of evil magic. Now, he feels he is no longer fit to lead the grove because they need someone so much more powerful and capable to help heal them and recover.
There you go. You're welcome, Larian. I solved your "Halsin is too weak mechanically to fit the story." Now you just have to actually have him explain it... or something similar... in the game itself.
