It’s not 100% reliable.
For this to work you need your opponent to have no board presence, threat or foresight, you also need Stealer of Souls and card draw such as Plot Twist (other cards are available) and then you are entirely at the mercy of animations, card draw and whether your opponent has made it impossible for you to play a health cost card by removing your immunity and bringing your health down to 3.
I often play Mecha’thun early and have a pit stop, watch him die, then play N’Zoth for the big reveal at the end.
EDIT: I give it a week before they nerf it.