So there are few lethal mutations on here and wanted to say (and here) others ideas for a lethal mute.
My lethal mute would be dragon like scales making the new cubbin a little hard to move.
This would be the messages for it-

Newborn stage- Your lioness has her litter of cubs and sees that one is very different from the rest. She is afraid for the young ones development.


1 month stage: Your lioness reports to you that the cub had difficulty breathing,eating and moving and fears the young one won't make it through the night.



2 month stage: Cub dies during the night due to complications. The pride held a greaves period roaring for Akuma to guide their cub.