This is a rule in my campaign. I never thought critical successes and critical fails had enough consequences in the rules. I have changed that.
CRITICAL SUCESS 20 BOONS
1 – NOTHING – Critical damage as normal.
2 – COMPROMISED DEFENSE – Target(s) takes -1 penalty to AC until the end of the attackers next turn due to defenses being damaged from their expert attack.
3 – LUCKY SHOT – Target(s) takes 1d6 extra damage.
4 – HAM-STRINGED – Target(s) is slowed to half movement until the end of the encounter DEX save ends. DC 10
5 – SWEEP THE LEG – Target(s) falls prone. No effect if target(s) is already prone or imobilized.
6 – BLEEDING OUT – You cut your opponent deep (or cause internal bleeding) Target(s) takes 1d4 damage each round. CON save ends.
7 – COMPROMISED OFFENSE – Your attack causes blood to trickle into your target’s eyes. Target takes -1 penalty to attacks and DEX saves until the end of your next turn.
8 – HAYMAKER – Your attack hits so hard it stuns your opponent until the end of your next turn.
9 – UNLEASH THE BIRDIES – Your attack hits so hard it dazes your opponent until the end of your next turn
10 – THAT STINGS – Target takes 1d8 extra damage
11 – LIGHTNING STRIKE – You attack so quickly that you get a free melee basic attack. If a spell was cast, the caster can use a basic melee attack OR cast one free cantrip. This only works on the creature you just attacked. No effect if the target dies.
12 – FLEET OF FOOT – You quickly attack your opponent and can shift up to your movement even if you have already used your move action. If another creature is within your movement you can make a basic melee attack on that creature. Spellcasters can make a melee or cast on free cantrip.
13 – NOT IN THE FACE – Your attack is so intimidating that one target can not take an action for one turn.
14 – THAT’S GOING TO LEAVE A MARK – The damage you do can’t be healed for the duration of the encounter. Effect is the damage is removed from the target’s maximum HP total.
15 – DISARMING STRIKE – One target drops weapon and must use a move action to pick it up. If target is a magic user, they may only use a standard action for the round.
16 – WHAT JUST HAPPENED? – Target automatically looses concentration. This affects concentration spells or any effect the target has used through spell or ability. They may not use Daily or Encounter abilities on their next turn.
17 – DID YOU SEE THAT? – Attacker gets combat advantage on next turn.
18 – FIGURED YOU OUT – Attacker gets +5 to attack on same creature (only one) next turn.
19 – DEVASTATING BLOW – Target loses 20% of current HP OR Crit Damage, whichever is greater – not both.
20 – DEATH BLOW – Attacker rolls d20 one more time. If they roll 20 again, the target(s) loses 50% of their current HP. If target(s) are below 10% of their maximum HP they are instantly taken to 0 HP but are considered unconscious. Player can roll a d20 for a third time. If they roll 20 for a third time the target is instantly killed regardless of an legendary action. If the third roll is not a 20 – nothing happens.
SPELL ADDENDUM – If the spell does not do damage in combat it will double the duration, range, burst, blast and / or effect.
CRITICAL 1 MISS DOOMS
1 – CLUNK – You damage your weapon and take a -1 penalty to attacks for the duration of the encounter until you can repair your weapon out of combat. If this was a spell you take -1 to DC.
2 – DISARMED – You drop you weapon and must use a move action to pick it up. If you have already moved this round you cannot attack and must use your move action in the next round to recover your weapon. If you were casting a spell it automatically fails and you loose your move action next round. Any concentration effects are immediately lost.
3 – LOOSE CONFIDENCE – You take disadvantage on attacks, saves and skill checks. WIS save ends DC 10.
4 – LEAVE AN OPENING – One enemy in melee range gets an opportunity attack.
5 – TRIPPED UP – You slip and fall. You are prone and must use a move action to stand up. This can be done in same turn if you have not used a move action yet.
6 – EMBARRASED – You take disadvantage on all saves and skill checks for the rest of the day.
7 – HURT SELF – You hurt yourself with the attack for 1d6 damage. This cannot take the the attacker to 0 HP.
8 – SHAKEN – Your are stunned until the end of your next turn.
9 – HAM-STRINGED – You damaged your foot in the attack and your movement is halved. DEX save ends DC 10.
10 – SHEEPISH – Take -5 to initative for the rest of the day.
11 – HURT SELF – You hurt yourself with the attack for 1d8 damage. This cannot take the the attacker to 0 HP.
12 – GET OVER HERE – Attacker is pulled one square.
13 – EXPEDITIOUS RETREAT – You must move your move speed away from target.
14 – MOCKED – Target laughs at you. Take -1 to AC and saves until end of you next turn.
15 – FLATFOOTED – Lose your movement until next the end of your next turn. You cannot move. Any advantges from total defense are lost.
16 – TAUNTED – Target taunts you. If you attack any target before the end of your next turn you will take 1d10 damage.
17 – EASY TARGET – You grant combat advantage until the end of your next turn.
18 – COWARE – Fear effect. WIS save ends DC 10.
19 – BROKEN – Take -1 to AC and saves. WIS save ends DC 10.
20 – NO ONE SAW THAT – Nothing happens.