I have no idea why this isn’t in the game. Most flagellant pictures you can find shows them with a heavy,2 handed flail. It should be his unique/exclusive/signature weapon.
it should have a slow,heavy hitting,long range,multiple target “light” attack combo
charged “throw”(as in he thrusts the pommel forward,sending the head flying in a straight line) and a special attack that is a 360 degree knockback/knockdown attack that drains stamina.
Not to mention he could have a heavy flail with ball & chain and one with a longer pommel and heavy head,similar to this one :