Zen Mode changes Darmanitan when it has half of its health or less. It changes from a pure Fire type into a Fire / Psychic dual type. Additionally, its stats change to be a bulky special attacker. If Darmanitan ever gets above half its HP, it reverts to its standard mode.
If Darmanitan loses Zen Mode or is suppressed with Gastro Acid, it will revert to Standard Mode even if it has below half HP. Even if another Pokémon gains Zen Mode, they will not transform. Zen Mode can not be Entrained or copied by Trace or Power of Alchemy.
The Zen Mode of Galarian Darmanitan turns it into a Ice / Fire type. Additionally, its stat increases attack and speed. Like the Unovan form, it will return to a pure Ice type when its HP is restored above half. According to the Pokédex, this form is caused by awakening Galarian Darmanitan's temper that it restores the flame sacs of the original species.