Did you leave the first map and go to the Rosymorn Monastery map? If you do that before resolving the Grove-situation, Halsin will die in the prison. Happened to me after I had freed him and then "parked" him in the prison so I could take care of the Goblin-leaders without him.
Usual order I do things in (which doesn't result in him dying):
- start the "Investigate Khaga"-quest
- Grab Mol's quest to steal the idol. Completing the Khaga-quest will stop the ritual and once it's stopped, Mol won't give you her quest any longer. Thus you will miss out on a pretty powerful reward.
- finish the Khaga-quest
- speak to Zevlor to get his first reward
- once the Druids stop their chanting, go and steal the idol to complete Mol's quest. It's a bit tricky, but totally doable. It's much harder (or maybe impossible?) to pull off if they're still chanting.
- go to the Goblin Camp and do all the quasi time-sensitive stuff there first (Loviatar's Love, free Volo, free the Owlbear Cub, etc).
- enter the prison and kill all the guards - probably best if you have speak with animals active so you can understand Halsin while he's in bear-form.
- after the fight, talk to Halsin and ask for his help/inform him of what's going on at the Grove. I always leave him there as I *think* if you take him with you, the whole camp will turn hostile once you leave the prison.
- start dealing with the leaders - I always trigger the Ragzlin-fight last as, AFAIK, attacking/killing him will turn everyone in the camp hostile.
- I usually take Gut out first (in her quarters) to gain access to her loot and to the secret passage into the Underdark. Great loot down there in the Selunite outpost.
- start killing all the Goblins outside - start with the guards you first run into (the group with the two Worgs), then deal with those in the courtyard. If all goes well, none of this will aggro any of the Goblins inside the temple.
- start killing everyone else (don't forget to free that guy in the cage for a SH-inspiration).
-Kill or knock out Minthara, but don't use the old rickety bridge/abyss method as she has some great loot on her you'll definitely want.
- kill Ragzlin last. Although you can easily loot his treasure room much, much earlier without triggering his fight. Just don't get too close to him and you're good.
Once all three leaders are dead, go back to the prison and speak with Halsin. He'll be super happy, transform into a rat and run off to the Grove. Go to the Grove and speak with him again, collect your rewards and that should settle the Grove-situation. Meaning you can now safely travel to the Monastery-map without killing Halsin off-screen.
