At the start of battle or at the end of a turn, if the HP of a Standard Mode Darmanitan with the Ability Zen Mode is below half, it will change into its Zen Mode. At the end of a turn, if the HP of a Zen Mode Darmanitan with the Ability Zen Mode is above half, it will change into its Standard Mode. Darmanitan is always in Standard Mode outside of battle.
If Darmanitan loses Zen Mode or Zen Mode is suppressed with Gastro Acid, it will revert to its Standard Mode and not be able to change form. If Darmanitan gains Zen Mode, at the end of the turn it will change form if its HP is below half.
Darmanitan's body turns into a stone-like statue that turns it into a part Psychic-type. When Darmanitan turns back to normal, its eyebrows become covered in flames and its body changes instantly back to Standard Mode, or Darmanitan's body becomes surrounded in red-yellow flames and it changes back into Standard Mode.