The simple, too-long-didnt-read analysis of Magic: The Gathering Arena is that its an online client and storefront for Magic , warts and all, as the game currently exists
we'll usually have ten lands out by turn 7, maybe earlier
On top of that, though, the card forces your opponents to reveal their entire hands to you, letting you see every card theyre currently holding
However, if you do, and that creature card is an illegal target at the time Mourner's Surprise tries to resolve, it won't resolve and none of its effects will happen
In the unusual case where Unwilling Vessel has +1/+1 counters on it and then receives an amount of -1/-1 counters that causes it to die, use the total number of counters that were on Unwilling Vessel when it died, including all of the +1/+1 and -1/-1 counters, to determine the value of X
The common play pattern of this will be using it offensively and soaking up the crack-back damage, but there will be times where you use it in a more tricksy way by using it as a fog, or even as removal and a fog