The only recorded case of someone not getting splatted by the Rainmaker happened at SplatKingdom IX.
Tomorrow night's Splatoon update, the first since 2.0, will add the Rainmaker mode to the ranked battles.
The objective is similar to Tower Control, in that the teams are trying to bring an object to the team's goal, but the titular Rainmaker allows the person holding it to fire Inkzooka shots. The weapon is set in the middle of the map and has a shield that has to be broken before it can be picked up. As well, the shield will explode when it's broken which can splat those in close proximity.
Rainmaker matches will have the same five minute time limit as other ranked battle modes, and a Tower Control-like scoreboard will be used as a tiebreaker.