Sun and Moon introduced S.O.S. battles, which occur when a Totem Pokémon or a wild Pokémon calls for an ally Pokémon to assist them. In general, Totem Pokémon call for allies on a frequent basis, while wild Pokémon are more likely to call for allies if they’re frightened or low on HP.
When this happens, your Pokémon will fight against two Pokémon simultaneously, exactly like a Double Battle (so the same mechanics apply) but you only control one Pokémon. Needless to say, S.O.S. battles put you at a numerical disadvantage–and you’ll need to be smart to thwart two opponents at once.
In addition to making battles more challenging, there’s a hidden layer of intricacy that may interest experienced Pokémon trainers.
So long as there’s only one wild Pokémon in battle, there’s a chance that the wild Pokémon will call for an ally Pokémon. This chance is increased if the wild Pokémon is low on health, you use the Adrenaline Orb item and/or your active Pokémon has the Intimidate, Pressure or Unnerve Abilities.
Therefore, you can encourage wild Pokémon to continue calling for ally Pokémon by knocking out one of the Pokémon, but keeping one of them standing, to preserve the status quo. It doesn’t matter if you keep the original or an ally standing–both are capable of calling for allies.
Which is a bad thing, right? The battle could theoretically go on forever. Well, yes, and this can be bad if you discover you’ve bitten off more than you can chew. If you want to stop the chain, you can cause a status condition that immobilizes your foe such as Paralyse, Sleep or Freeze.
Actually, it turns out you want to continue the chain. The higher the chain, the more likely you’ll find Pokémon with high IVs, a rare Hidden Ability or even Shiny Pokémon! In addition, Effort Value (EV) gain is doubled when knocking out ally Pokémon, making EV training a doddle with the right preparation.
You can use False Swipe or Hold Back to avoid accidentally defeating the Pokémon before it has called an ally and the Adrenaline Orb item to encourage the Pokémon to do so. Whenever the Pokémon fails to call an ally, you can continue using the Adrenaline Orb–doing so won’t do anything, but it won’t expend a use, allowing you to stall for time.
Depending on the wild Pokémon that’s calling for help and your current location, sometimes an ally Pokémon of a different species may appear instead. This can be an evolved version of the original Pokémon, such as Bagon calling for Shelgon or Salamence.
Additionally, there is a special type of S.O.S. battle that can yield rare and exclusive Pokémon, known as weather S.O.S. As its name suggests, it occurs when a weather is in effect–namely rain, sun, hail and sandstorm. During these conditions, certain rare Pokémon may appear as ally Pokémon.
As with regular S.O.S., Pokémon exclusively found during weather S.O.S. are location-dependent on top of weather-dependent. Usually, it’s easier to wait for the weather to naturally occur in those locations, but you can use Pokémon moves to create that weather as well.