Smoke Screen on a Rift Herald then Gust on the same reaction chain, does the Gust resolve before the Smoke Screen and make the Rift Herald not mighty? If not, can you react with sacrifice to kill the Rift Herald before Gust resolves?

To determine the resolution of this chain, we must follow the Last-In, First-Out (LIFO) rule for chain resolution.

The Chain Sequence

1. Smoke Screen is played targeting your Rift Herald. It is added to the chain.
2. Gust is played as a reaction, also targeting Rift Herald. It is added to the top of the chain.

The chain currently looks like this (from bottom to top):

1. Smoke Screen
2. Gust (Top)

Resolution

• Does Gust resolve before Smoke Screen? Yes. Because Gust was added to the chain after Smoke Screen, it is the topmost item and resolves first.
• Will Rift Herald be returned to your hand? Yes, provided Rift Herald has 3 or less Might when Gust resolves.
• Will Smoke Screen's effect apply? No. After Gust resolves, the Rift Herald will have been returned to your hand. When Smoke Screen attempts to resolve, it will fail to find its target (it has changed zones to a non-board zone). The instruction will be ignored (Rule 359.3.e.5).

Can you sacrifice Rift Herald?

If you have an ability that requires sacrificing a unit as a cost (e.g., a "Kill a friendly unit" effect), you can indeed activate it while the chain is in a closed state.

Sequence:

1. Chain: Smoke Screen (bottom), Gust (top).
2. You activate a sacrifice-based ability in response to Gust. The chain is now: Smoke Screen, Gust, [Your Ability].
3. [Your Ability] resolves first: You sacrifice Rift Herald to pay the cost.
4. Gust resolves next: Because its target (Rift Herald) has left the battlefield and is no longer a legal target, Gust "whiffs" (does nothing).
5. Smoke Screen resolves last: It also fails to find its target (the unit is in the trash) and does nothing.

Summary:

• Gust resolves first. It will bounce the Rift Herald to your hand if it has 3 or less Might.
• You can sacrifice the Rift Herald in response to Gust. If you do, both Gust and Smoke Screen will fail to resolve because their target is no longer on the battlefield.

This is my interpretation based on the mechanics of the LIFO chain and targeting rules. No official FAQ exists for this specific interaction.