#fizzling

In anticipation for the Phyrexia: All Will Be One pre-release, I've been chewing over each card in the set and noticed something in the release notes on Basilica Skullbomb that I don't quite get: "If the target of Basilica Skullbomb's last ability isn't a legal target as the ability tries to resolve (probably because it has left the battlefield), you won't draw a card."

Can you explain why this is the case? Thanks!

When a spell or ability with one or more targets goes to resolve, it checks its targets again to see if they're still legal. As long as at least one of its targets is still legal, the ability will resolve and do as much as it can to its remaining legal targets. However, if none of its targets are legal when it goes to resolve, then it won't resolve at all and will be removed from the stack without any effect. This means that any secondary effects that don't care about the targets won't happen either. This is sometimes informally called "fizzling".

Once a spell has been cast, it doesn’t check to see if its targets are legal again until it starts to resolve. Only then will it leave the stack if all of its targets have become illegal.

In the scenario you describe, Negate will counter the first Lightning Bolt and Baral’s ability will trigger since that Lightning Bolt doesn’t yet know that its target isn’t there anymore.