If I hidden blade a rift herald do I draw first or summon a card from my hand first

You draw the 2 first. Hidden Blade's "draw 2" is part of its own resolution, and the Deathknell trigger can't resolve until Hidden Blade has fully finished resolving (Rule 158.3).

Sequence: Hidden Blade resolves → "Kill a unit" → Rift Herald dies, its Deathknell goes Pending on the chain → Hidden Blade's next instruction "Its controller draws 2" executes → Hidden Blade finishes → the Deathknell trigger then resolves and you play a unit from your hand to your base, ignoring its Energy cost.

So yes, you can draw into a unit with Hidden Blade and then immediately play it via Rift Herald's Deathknell.