Have you ever wondered why, for all the urgency of having to leave and the imminent threat of assassins, the main character of Baldur's Gate is at leisure to stroll across the grounds of Candlekeep, chatting, delivering bottles of cow cure, exterminating pests, even resting at the inn for days and weeks at a time if he has a notion to? Of course, the out-of-character reason for this back in the day was that the Prologue was a tutorial and only intended to teach players the controls and basic how-tos of gameplay. Nowadays, however, hardly anyone perceives it that way or needs to be instructed. Most players probably go from task to task for a little starting experience, if they do not find them too boring, before setting out at some time.
This mod brings the Prologue back in-character and makes the urgency very real. You have one hour - sixty game time minutes - to finish your business in Candlekeep and depart with Gorion; and if you do, nothing will happen. One minute longer, however, and very real killers will show up wherever you are and whatever you may be doing - pick-pocketing idle nobles, fighting illusionary monsters, listening to prophesies of Alaundo and so on. All of those activities are very nice, but other people living it out in the monastery are not in the unique situation of being on a kill list. These assassins are much more competent than the pathetic tutorial-type thugs you already know. For a 1st-level character without a party fighting them means almost certain, but not absolutely certain, death.
Most players and their characters will just have to pack it on time. If you dig your heels down and take them on, however - and defeat them - you will be treated to a little loot and some new content. Read the dialogues attentively. The most obvious reward is a new 1st-level spell, Vanishing Act, carried by one of the assassins (you will wish you had it before meeting them). If the battle turns out too difficult for your character and you end up having to run away after all, there are two other places where you can get such a scroll. One is far across a sea and another is in the home of the enemy.
Taking the assassins on single-handedly is probably too much for any character, but Candlekeep is still your home after all. Think about who might help you. In the time that you have give people a reason to intervene. It also helps to be well-liked to begin with. Of course, not everyone will lay down their lives for you even then.
In addition to all this, I took a hammer and pliers to some other things around Candlekeep. The spell Firebead Elvenhair casts on you to "protect you tonight" will really last into the fateful night, and so on.