Dishonored – How to Complete Optional Tasks Later

Violent solutions to problems and a high body count increase the level of Chaos in the city.

When the city is a violent chaotic place, people react accordingly. There are more guards and more traps. There are more rats to take advantage of the abundance of food. Essentially, if you signal to the game that a fight is what you're after, then a fight is what you'll get. The final mission, in particular, is dramatically altered by the level of chaos you've engendered or avoided up to that point.

Additionally, Dishonored has several endings, and chaos is the primary determinant of which of these you'll see.

First of all, if you pick up all the runes, you should be able to grab Blink II, Agility I and Possession I by the end of the first mission. All three are enormously useful, however, within that first mission, I'd have to say the most valuable pickup is Blink II. You didn't mention it, but it really is the single most valuable skill I've found for stealth runs. After that, choosing between Agility and Possession is mostly a matter of taste. I'd favor Agility personally, but that has more to do with my fondness for blink spam and distaste for using Mana on anything else than it does to do with any difference in the usefulness of the two skills, and you can easily grab Possession II quite early on in the second mission, where I did find it pretty useful.