If I conquer with Kai'sa and my legend is Draven, the opponent has Scuttle Crab there. So will I draw 2 first or deathknell trigger first?

No — the Deathknell resolves before any of your draws.

Sequence during the Combat Showdown (Rule 461.1–461.3, per FAQ #10347):

1. Scuttle Crab's Deathknell fires during Combat Cleanup and resolves first — your opponent reveals their hand, and you gain 1 XP.
2. Draven, Glorious Executioner ("When you win a combat, draw 1") triggers when the combat result is determined — you draw 1.
3. Conquer happens — since Kai'Sa survived, her "When I conquer, draw 1" triggers — you draw 1 more.

So you end up drawing 2 total (1 from Draven, 1 from Kai'Sa), but both happen after the Deathknell reveal, in separate FEPR windows. The reveal never sees either drawn card.