If a unit with Deathknell gets killed but is saved by Zhonyas, does the Deathknell effect still trigger?

Ruling: No, Deathknell does not trigger. Since Zhonyas prevents the unit from dying by replacing the death with a recall, the unit does not die and therefore Deathknell does not activate.

Nuances:

• Zhonyas is phrased as "next time a friendly unit would die, kill this instead" which means the protected unit never actually dies
• The game effect of dying is replaced with a recall