If I have a Rift Herald alone at a battlefield and I use Sacrifice on it, can I play the card from Rift Herald's Deathknell to that battlefield?
To answer your question, we must look at the specific text of the cards and the rules regarding battlefield control and Deathknell.
The Short Answer
No, you cannot play the unit from the Rift Herald's Deathknell effect to that battlefield.
The Reason
The primary issue is where the Rift Herald's Deathknell effect allows you to place the unit.
- Card Text: The Rift Herald's Deathknell ability explicitly states: "Play a unit from your hand to your base, ignoring its Energy cost."
- Placement Restriction: Because the card text specifically dictates "to your base," you are restricted from playing that unit to a battlefield. It must enter at your base.
Regarding Battlefield Control
Even though you are using a spell (Sacrifice) to kill the Rift Herald, the rules regarding battlefield control generally allow you to retain control of a battlefield while an item is on the chain (Rule 187.4.c and related FAQs). However, this protection only helps you play a unit to the battlefield if the Deathknell effect itself allowed for that placement. Because Rift Herald specifically designates "your base" as the destination, you must follow that instruction, regardless of whether you control the battlefield.
This is my interpretation based on the card text of Rift Herald and the mechanics of ability instructions. No official FAQ exists for this specific placement restriction interaction.