It will be a dodgeable execute, so things like Zhonyas, Flash, Fizz E, Kindred ult etc. would be able to negate it.
Right now the current Elder drake is probably actually stronger than this proposed version, as this one only comes with the execute and not the dragon stacking.
I wanna wait until we see the actual numbers, but I am thinking it will be like 10-15% which I think is reasonable as it is a big objective and it gives the team that gets it a big advantage that you need to be aware of when fighting and a chance for teams who are behind (who maybe didn't get dragons earlier) a chance to get back into the game.