Grass Pledge inflicts damage. It has a regular power of 50.
When two allies attempt to use Grass Pledge and either Fire Pledge or Water Pledge on the same turn, the ally moving first will not itself use a move, but instead the ally moving second will use a combined attack with a power of 150 and an additional effect immediately after it:
If Grass Pledge and Fire Pledge are used in the same turn, the ally moving second will use Fire Pledge with a power of 150, and create a sea of fire on the target's side of the field for four turns. The sea of fire damages all non-Fire-type Pokémon on that side of the field for 1/8 of their maximum HP at the end of each turn.
If Grass Pledge and Water Pledge are used in the same turn, the ally moving second will use Grass Pledge with a power of 150, and create a swamp on the target's side of the field for four turns. This swamp quarters the Speed of all Pokémon on that side of the field.
If a Pokémon uses one Pledge move and a partner has selected another Pledge move, but none of its partners are able to use their selected Pledge move that turn (e.g. paralysis, Sky Drop, etc.), no move will be executed.
Grass Pledge is unaffected by a held Grass Gem (and will not consume it).
Generation VI onward
Grass Pledge's regular power changed from 50 to 80. In addition, Grass Pledge does not work in Sky Battles.
It damages an enemy. If it's used with Fire Pledge in an Alliance, the enemies in the same room will get the sea of fire condition, and if it's used with Water Pledge, they will get the swamp condition.