AceCalhoon and anaximander have both provided some well thought out and thoroughly explained answers here already. Ace makes a case for both possibilities equally, and anax makes a case for instant death checking before reverting to normal form. I will make a case that instant death is checked after a reversion to normal form.
SO, for the third time here, let us look at the relevant Wild Shape text:
- You Automatically revert if you fall unconscious, drop to 0 hit points, or die.
- When you transform, you assume the beast's hit points and Hit Dice. When you revert to your normal form, you return to the number of hit points you had before you transformed. However, if you revert as a result of dropping to 0 hit points, any excess damage carries over to your normal form. For example, if you take 10 damage in animal form and have only 1 hit point left, you revert and take 9 damage. As long as the excess damage doesn't reduce your normal form to 0 hit points, you aren't knocked unconscious.
I would like to point out here and now what really sells me to my case:
For example, if you take 10 damage in animal form and have only 1 hit point left, you revert and take 9 damage.
From what I can tell, this reads like the following:
- The Druid is in animal form and has 1 hit point left.
- The Druid takes 10 damage while in animal form.
- The Druid is dropped to 0 from taking 1 damage, so they revert to their normal form. Their hit point total remains at 0.
- The Druid is now in normal form, and must take the remaining 9 damage.
- Instant Death is checked here.
Regarding AceCalhoon's own answer, this would be applying the second interpretation of the Instant Death rules. The reason that you would revert in step 3 is because you are taking enough damage to drop you to 0 hit points, at which point the hit point reduction is interrupted by the Druid reverting to their normal form, and then the hit point reduction can continue here.
So to spell it out clearly, and to answer the actual question, let us look at the actual scenario under these circumstances:
- The Druid is in animal form, and has 1 hit point, being a spider.
- The Druid is hit for 2 damage.
- The Druid is reduced to 0 hit points by taking 1 of that damage, and reverts to their normal form at 0 hit points.
- The Druid takes the 1 remaining damage.
- Instant Death is checked here, and unless the Druid honestly has 1 Max Hit Point, they will not die from this attack.