RollQuest summary of the SRD 5.1 rules, not a quotation.
Pick one: Colossus Slayer: once per turn, when you hit a creature with a weapon attack, it takes an extra 1d8 damage if it’s below its hit point maximum; Giant Killer: when a Large or larger creature within 5 ft of you hits or misses you with an attack, you can use your reaction to attack it immediately after its attack, provided that you can see the creature; or Horde Breaker: once on each of your turns when you make a weapon attack, you can make another attack with the same weapon against a different creature within 5 ft of the original target and within your weapon’s range.
RollQuest ships the D&D 2014 SRD as a content pack, on a 5e character sheet that does the maths - roll from the sheet and let the numbers work themselves out. Free to start.