Zombuul vs Assasin

Just as the title suggests, what would happen if the Assasin targeted the Zombuul?

My groups consensus is that the Zombuul dies for real and the game is over and good wins.

This is assuming everyone is sober healthy, and nothing else is preventing the game from ending.

The second conclusion my group made was trying to apply a mtg based layers system which got out of hand quickly. Replacement effects are weird.

The other option is the Zombuul “dies” and is now registering as dead so the Assasin’s ability believes it worked properly.