There is an optional rule in the DMG (Dungeon Master’s Guide – p273)) – It deals with a creature or monster taking more than half of its total hit points in damage from a single source (hit or spell).
If that happens, the creature must make a DC 15 CON save. On a save, they take just the damage indicated. On a fail, they also roll on the System Shock table on p273 of the DMG.
Roll a d10:
1: creature drops unconscious to 0 hp and makes death saving throws
2-3: creature drops unconscious to 0 HP and is stable (no saving throws needed)
4-5: creature is stunned until end of its next turn
6-7 : creature cannot take reactions, and has DISadvantage on attack rolls and ability checks until end of its next turn
8-10: creature cannot take reactions until its next turn.
Try this out for your next game and make those critical hits really be critical!