At Metagame they have the semifinals with Jacob Hershberger vs. Milton Figueroa. In game one Jacob uses a Nasty Surprise and an Overload to ready his Mag's in response to Heroic Sacrifice. How is that legal? I thought that before a attack concludes all effects must resolve and if there is no longer a defender then the attacker readys. If that is the case how does Heroic Sacrifice fizzle?

Heroic Sacrifice Goes onto the chain. In response, Nasty Surprise and overload are played.

The resulting resolution looks like this:

Nasty Surprise Resolves.
Character gets +5 Attack.
Then overload resolves.
Character is stunned.
Heroic Sacrifice Checks for a legal target upon resolution. Since there is no longer a legal defender, the effect "fizzles"

If there is no legal target at the resolution of an effect, then the effect is negated. Heroic Sacrifice targets a Teen Titans defender, and stunning the defender before Heroic Sacrifice's resolution negates its effect; it is no longer a defender, and so is no longer a legal target. The attack will conclude with no defender, but with Magneto still an attacker, and Magneto will ready.

Your right, I forgot you lose the defender characteristic when stunned.