it depends who the person owning that 18 million ETH tokens is!
- an imaginary person: obviously no effect
- a whale who is not connected to ethereum foundation and Vitalik: price can take a hit but those guys have a lot more money to put up buy walls that one whale can do nothing.
- the ethereum foundation and Vitalik themselves: price will tank hard and there is nothing stopping it because the only one that was preventing crashes so far with their fake-whaly-buy-walls are selling themselves now.

so figure out who the whale is and you get your answer.