When a Pokémon with Desolate Land enters the battle or a Pokémon gains the Ability Desolate Land, the Ability creates extremely harsh sunlight, a unique variation of harsh sunlight. In a Rotation Battle, Desolate Land will not activate if the Pokémon is sent out in a rotated-out position, nor will it activate if the Pokémon is rotated in. In addition, Desolate Land will not activate if extremely harsh sunlight is already present on the battlefield.
Along with the usual effects of harsh sunlight, extremely harsh sunlight causes damage-dealing Water-type moves to fail (unless a Pokémon on the field has the Air Lock or Cloud Nine Ability). Additionally, extremely harsh sunlight causes the weather-modifying moves Sunny Day, Rain Dance, Sandstorm, and Hail to fail if used, and causes the Abilities Drought, Drizzle, Sand Stream, and Snow Warning to fail to activate.
Once in effect, as long as there is a Pokémon on the field with the Ability Desolate Land active (even if it is not the Pokémon who created it), the harsh sunlight will remain in effect unless it is replaced. (If the only Pokémon on the field with Desolate Land is rotated out or switched out, or has its Ability suppressed or replaced, the harsh sunlight will dissipate.) The harsh sunlight can be replaced by Primordial Sea or Delta Stream.
Outside of battle
Desolate Land has no effect outside of battle.