Mark. This is really fun. Requires hitting the target, so it's not like your rogue can just OA every creature on the field and deal full sneak attack damage every turn, or whatever, but it does allow a "tank" to be "sticky", especially combined with other existing defender options. Why don't more people use this? It's even more effective in the hands of a fighter than anyone else at high levels, but also allows Barbarians and Paladins, the other two classic defenders, to really punish enemies to who try to circumvent them, all without giving up other reactions like Protection Fighting style.