Automatic Attacking (5e Variant Rule)
From D&D Wiki
How to Attack
All attacks (weapon and spell) always hit, and always deal average damage plus X%, where X is your to-hit bonus times 5 (I'm sure there's a more elegant way to write that equation, but I can't think of one).
All characters get a number of "Armor-Class-Temporary-Hitpoints" equal to their number of hitpoints times ten less than their armor class, all divided by twenty. ACTHs are lost after temporary hitpoints but before real hitpoints. Whenever a character regains hitpoints, they also regain the same number of ACTHs.
Damage dealing effects that don't involve an attack roll deal 25% more damage.
When to Attack
The Attack action can't be taken using your action. Instead, once per turn you may take an attack action to Attack or Cast a Spell. All spells with a casting time of 1 action instead have a casting time of 1 attack action, unless the DM says otherwise.