The best way to take advantage of Threaten effects is to sacrifice the creature you steal so that you remove their creature while generating extra value
Now, I finally get to truly see it as it should be seen
For black, as with Frogkin Kidnapper, its a temporary discard effect
I played against Jadine Klomparens in round one and immediately realized how badly I was going to get destroyed when I didnt follow up with a turn 2 play against her Jeskai deck, and she proceeded to best me down with a Mantis Rider, while my Brimaz stared helplessly from the ground
For example, if you put Null Profusion (an enchantment that says your maximum hand size is two) onto the battlefield and then put Venser's Journal onto the battlefield, you'll have no maximum hand size
If the copied creature is copying something else, then Naga Fleshcrafter enters as whatever that creature copied