Q304

If you have temp HP (lets say 25 from a 5th lvl Armor of Agathys) and take damage (lets say 24 damage, so the temp HP absorbs it all), do you have to make a concentration saving throw for a spell you are concentrating on? What is the DC? Where can I find this covered RAW?

Do you take 0 damage so there is no save required? Do you "take" 24 damage but your actual HP is unchanged so there is a 12 DC con save?

I'm sure this has been covered before but I can't find it. Thanks.