If you Sacrifice as a reaction to Hidden Blade, do you draw 4 and channel 1 rune exhausted? Or just draw 2 and channel 1 rune exhausted for Sacrifice?

If you play Sacrifice as a reaction to an opponent's Hidden Blade, you will not draw 4 cards. You will only draw the 2 cards granted by the resolution of Sacrifice.

Here is the breakdown of why this happens:

The Sequence

1. Opponent plays Hidden Blade: They target your unit and place it on the chain.
2. You react with Sacrifice: You pay the cost by killing your own unit (the same one targeted by Hidden Blade).
3. Sacrifice resolves: You draw 2 cards and channel 1 rune exhausted.
4. Hidden Blade attempts to resolve: It checks its target. Because your unit was already killed by the resolution of Sacrifice, it is no longer at the battlefield.

Why you don't draw 4

• Hidden Blade's Draw: According to RiftJudge FAQ #3700, if you kill your own unit in response to an opponent's Hidden Blade, the opponent's Hidden Blade loses its valid target. Because the unit no longer exists at the battlefield, the game cannot determine its controller, and the draw effect of the opponent's Hidden Blade fails to trigger.
• Sacrifice's Draw: You successfully resolve Sacrifice, which grants you its own draw of 2 cards.

Summary: You get the 2 cards from your Sacrifice, but your opponent gets 0 cards from their Hidden Blade because their spell's target was removed before it could resolve.