Ok so I was wondering how this works. I have a maester of the sun in play and my opponent valars. In response to the valar I save a character using the maester, and discard Darkstar as the cost of activating it. I then put Darkstar into play. Does Darkstar live because Valar has already triggered?

Leaving aside the fact that Valar is not considered to be "triggered," this question went back and forth before being settled in FAQ 3.41:

(3.41) Cards entering play between initiation and resolution of an effect

If a card enters play after the initiation of a general (non-targeting) effect, but before that effect's resolution, it may be affected by the general effect.

Since Valar is a general, non-targeting kill effect, Darkstar will be killed by the resolving effect when he enters play after Valar's initiation but before it's resolution.