­Culling Mark as a Geass. For the green common "negative" Geass slot.

This will never be an all-star, but it showed it had value in playtesting. Making a creature have to engage provides a way to turn on the combat tricks in your hand.

The Gatherer comments on Culling Mark do reckon it's extremely overcosted though. This could perhaps be a Geass of Provoke instead.

This at least could very easily become an Aura - "Enchanted creature blocks each combat if able."

And all the tricks you've exiled aside.

Regarding making it permanent - could do; doesn't make much odds though - it'll still be used to kill the target immediately. I guess it works well in case you fail to kill it. Also adds the flavour of the geass being permanent.