As far as I know, it'll add to the damage, but against most bosses you'd already be hitting the damage cap with max hp attacks anyway, so no gain there.

I'd go with soul reap instead if you don't want cruelty

As a twist Lightning Cage is really good on Marius if youre using him in arena.  Of course won't add to damage but it's anothe rreasonable option.

Thanks

Phantom Touch is basically the other option if you're not looking at arena.