#target legality

[magicjudge]

My opponent casts Banefire, targeting me for lethal damage. In response, I cast Aegis of the Gods (with flash due to some other effect). I am now hexproof. After it resolves, my opponent kills Aegis with a Lightning Bolt. Am I still being targeted by Banefire? Is the legality of targets checked continually, or just when the spell goes to resolve?

You're still a legal target for Banefire and will be roasted to a crisp soon enough.

The legality of a spell or ability's targets are only checked twice: The first time is during the casting process, where they must all be legal for the spell to be able to be cast / the ability to be activated. The second time is when the spell or ability is about to resolve.

They aren't checked at any point in the interim, so even though you wouldn't have been a legal target for Banefire had it resolved while Aegis was still on the battlefield, that doesn't matter because neither of its legality checks happened in that window of time.

[magicjudge]

When do spells fizzle? Example player A cast dreadbore targeting a creature, and his opponent responded by casting twincast and directing the copy to a planeswalker (controlled by player A). Player A then cast terminate targeting the creature dreadbore was targeting in an attempt to make dreadbore 'fizzle' since it no longer has a valid target. Does twincast still work here? Can it still copy dreadbore and destroy the planeswalker? Thx

Whether or not a spell or ability’s targets are legal is checked only twice: First when you cast the spell and then when the spell is about to resolve. In between, the spell or ability doesn’t care if those targets are legal or not and will remain on the stack in blissful ignorance during that time even if all of its targets are now illegal and it will be countered on resolution (a.k.a. “fizzle”). A spell with multiple targets only fizzles if ALL of its targets are illegal when it tries to resolve.

In the situation you describe, the Twincast will resolve and copy the Dreadbore. Dreadbore’s target is illegal, but Dreadbore doesn’t find that out until it tries to resolve later on. The new copy of Dreadbore will then resolve and destroy the targeted planeswalker. Finally, the original Dreadbore will try to resolve and find out that its target is illegal. Upon checking its target again, Dreadbore will be countered as it tries to resolve.