How does Hall of Legends Battlefield work with Rek'sai Legend?

Here's the ruling on Hall of Legends and Rek'Sai (Void Burrower):

No, Hall of Legends cannot ready Rek'Sai to let it trigger its own ability on the same conquer. If Rek'Sai is already exhausted when the conquer occurs, its "When you conquer" ability simply does not trigger — it never goes on the Chain (FAQ #11259). Hall of Legends readies after the fact, but the trigger window has already passed.

However, Hall of Legends is still useful with Rek'Sai in a different way. If Rek'Sai is ready when you conquer Hall of Legends, both triggers fire. You can order them so that Rek'Sai's ability resolves first (exhaust it, reveal, play a card), then Hall of Legends resolves — letting you pay 1 to ready Rek'Sai for the next conquer. This works fine.

The key distinction: Rek'Sai's ability requires it to be ready at the moment the conquer event occurs in order to trigger at all. Since it's a triggered ability with an exhaust cost baked in ("you may exhaust me"), if it's already exhausted when the condition is met, the game doesn't offer you the choice — the trigger never enters the Chain. Hall of Legends readying it afterward comes too late for that same conquer instance.