When you attack a creature with this magic weapon and roll a 20 on the attack roll, that target takes an extra 3d6 necrotic damage, provided that the target isn’t a construct or an undead. You gain temporary hit points equal to the extra damage dealt.
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.