What happened?

[profntbtr]

i doubt it's a mantis, a mantis's attack is designed to kill, and is usually very effective. if you've seen it alive with damage, it is not a mantis. i'd suspect the other fish, but as mike said, it isn't the lion, because lions don't really attack as much as consume. i'd say the other anthias are to blame, sometimes fish refuse to play nice. if you can i'd separate the injured fish and see if things get better. it is also possible you were sold a pair of males and a female, which could prove problematic as anthias are haremic.