So at level 20 you get a flat 2d6 per attack with no resource expense and no drawbacks. This is too much. As much as I hate to say it, there's a reason why HM has concentration: because it's a flat damage multiplier with drawbacks. If you remove concentration from it, then it becomes free damage for anywhere between 1-24 hrs that the DM cannot mitigate, because instant spell effects cannot be counterspelled or dispelled once they set in. All damage multipliers that have a duration longer than a minute require concentration, and before you mention Divine Favor--it's a 1d4, it lasts a minute and requires using spell slots after every encounter, which a Paladin might use for smites instead.