The map its mobs and the boss is generated when someone enters it.
If this is the highest tier, that is, plat tier, there is a high probability that Ursoth will come out, but it is not 100%, Helena or one of the other 2 weak bosses can still come out.
Having a higher tier only gives more chance of a better boss coming out.
Easily testable, only run in a ghost party. I do it often when I can't find a party.