The Shield Master feat says that (emphasis mine):
If you are subjected to an effect that allows you to make a Dexterity saving throw to take only half damage, you can use your reaction to take no damage if
you succeed on the saving throw, interposing your shield between yourself and the source of the effect. -PHB 170
If this ability of the feat is used against, for example, (a) the burning hands spell or (b) the breath attack of a dragon, would this feature of the feat shield you from it (providing you make the save)?
If it does, Would this also protect anyone directly behind you? For example, would this protect people to the sides of them if they are far enough back (think like air flows around the tip of an airplane)?
Best Answer
Yes, it will protect you. Unless...
you are using the Facing variant rule from the DMG (pp 252). But even then, it's more of a DM call on whether or not the Shield Master would activate.
Your bolded line of the feat description is more of what it seems to be doing, but the primary lines are that it activates when you go to make a Dexterity Save against an effect.
No, it will not protect anyone else
Unless your DM changes the feat to work like that, then it only protects you. The language of the feat is clear that the Mastery only helps you reduce damage and does nothing for anyone else.