Very simple addon that warns you when you are below 50% health by flashing the borders of the screen red and playing a Murloc sound. It will keep doing this on taking damage until you are above 50%. It will not spam it though, but require 5 seconds to pass between each warning.
I found it useful when doing Sartharion + 3 drakes for dealing with Twilight Torment.
Not recommended for tanks unless they are really fond of Murlocs.