Can I conquer with Phoenix with Assault, then on conquer use Reksai Legend, can I then use Sacrifice on Phoenix as a reaction to Reksai Legend to kill Phoenix, draw cards from Sacrifice, and then reveal from Legend? Or do I lose Assault before I can do all that?

To clarify your scenario, let's break down the timing and the game mechanics involved:

1. Does the Assault bonus expire?

No, your Assault bonus remains for the duration of the combat. According to Rule 807.1.d.1, Assault remains in effect as long as the unit maintains the Attacker designation. Even after the showdown transitions from the "Initial Chain" (where you conquered) to an "Open" state, you are still in the combat phase. The Attacker designation expires during the "Combat Cleanup" step, which occurs after both players pass priority in the combat showdown.

2. Can you use Sacrifice as a reaction?

Yes, you can, but there is an important timing constraint:

• The Chain: When you conquer, your "When I conquer" trigger (or any conquer effect) goes on the chain.
• Reaction Timing: You are allowed to play Sacrifice as a reaction to the Rek'Sai ability (if it was added to the chain) or as a response to the conquer event itself, provided you have focus.
• Resolution Order: Because Riftbound uses LIFO (Last In, First Out) for the chain, if you play Sacrifice after the Rek'Sai ability is on the chain, Sacrifice will resolve first.

3. Will this work as you intended?

There are two critical issues with your proposed sequence:

• The "Reveal" Timing: According to Rule 424.3 and the related FAQs, when you use a "Reveal" effect (like the Rek'Sai legend), the reveal happens as part of the resolution of that ability. If you kill the Phoenix with Sacrifice before the Rek'Sai ability resolves, that is fine; however, you cannot "wait" to reveal the cards until after Sacrifice finishes and then decide to reveal. The Reveal is an instruction within the Rek'Sai ability.
• Ordering Triggers/Effects: If you are the controller of both the Conquer event and the Rek'Sai ability, you choose the order they go on the chain (RiftJudge FAQ #9838). If you put the Rek'Sai ability on the chain first, then the Conquer trigger second, the Conquer trigger will resolve first. You cannot insert an action in the middle of resolving a single chain item.

Summary Ruling

1. Assault: You do not lose your Assault bonus until the Combat Cleanup step (which happens after all chain interactions are finished). You are safe to perform your actions.
2. Sacrifice and Rek'Sai: You can play Sacrifice to kill your Phoenix as a reaction. The Phoenix will trigger its "Deathknell" (if it has one) or allow you to use its recursion ability (as confirmed by RiftJudge FAQ #10252, if the conditions are met).
3. The Reveal: You must resolve the Rek'Sai legend ability in its entirety once it hits the top of the chain. You cannot "pause" the resolution of the Rek'Sai ability to play Sacrifice halfway through its instructions. You must play Sacrifice while the Rek'Sai ability is waiting on the chain.

Important Note: As per RiftJudge FAQ #9215 and #9823, remember that when you play a card revealed by the Rek'Sai legend ability, you must pay its full cost.

This is my interpretation based on the mechanics of the chain and combat phases. No official FAQ exists for this exact multi-card interaction.