What happens when an attack with a raider drops a hero to 0 hp? [duplicate]

So, imagine you are hit by a monster and dropped down to 0 hp. But then the same attack also requires you to make a Constitution save or take extra damage. Is this attack treated as a single damage and you stay at 0 hp, or what I think, you are dropped to 0 hp from the normal attack, then you make a Con save and if you fail it, you take the extra damage causing the character to receive a failed death save?

Edit: As my question was deleted because there is a similar one, but there is no answer to this exact circumstance. Here we have one attack that requires an attack roll for the 1st portion of damage and then a save for the additional damage. I cannot find an answer that explains this situation!