When can I use Bastion of Mental Clarity?

Question

If I use Bastion of Mental Clarity as an immediate interrupt, does it pump my will for the attack that just happened or is it in effect afterwards?

Answer 1

It works in time to bump your Will.

As immediate interrupt, the effects of Bastion of Mental Clarity^DDI take place before the event described in its trigger occurs.
This is a prerogative of immediate interrupts, and it's fairly common to see defensive powers working that way.

Beware that creatures are limited to 1 immediate action (either interrupt or reaction) per round of combat.