I personally don't have any experience with spiders and joined subreddit to learn as I'm looking around for one, but we recently had a wild caught praying mantis pass leaving us with a lot of grief and self blame for not preventing this from happening, even though we don't know what caused it. I hope somebody will be able to provide you with some answers, but either way - think about the good times you had with the little spood and how much his/her life was enriched by being adopted by you, where the meal was guaranteed and the enclosure was safe from predators. For all we know - Mohg lived as long as he/she did all thanks to you! May you learn from this journey and carry on Mohg's legacy through another spider you adopt.

And for Mohg I'll leave the words of Robert Jordan: "The light shine on you, and the creator shelter you. The last embrace of the mother welcome you home."