Black Surgeonfish

Found in schools, in open areas close to corals and rocks.
Western Indian Ocean - Gulf of Aden to Red Sea

Surgeonfish have a blade like spine in the tail that points outwards when bent, unicorns have two hook-like plates along the tail, these are used for defence and are as sharp as a surgeons scalpel, hence the name "Surgeonfish".
Some species are venomous.