Make Every Boss Legendary
For D&D 5th Edition, by Tales of Hermes
Version 1.0, Sept 2nd 2022 - works with D&D up to Spelljammer!
When it comes to boss battles, a lot can go wrong, a DM might take a monster that is either way too powerful or way too weak, which in both instances leads to an unsatisfactory battle. But the monster itself might also not be the best fit to begin with. Some monsters have a very high challenge rating because of something else than just dealing damage, if you chose such a monster to be the Big Bad Evil Guy you might have a disappointing surprise on your hands.
A Boss can take a number of legendary actions equal to the number of players it has to face minus 1. Only one legendary action option can be used at a time and only at the end of another creature's turn. The Boss regains spent legendary actions at the start of its turn.
Damage & Save DC
If a legendary action does damage or needs a save DC, but the Boss itself has no features that do damage or need a saving throw, use these calculations instead.
Damage = the Boss' CR x 5
Save DC = half the Boss' CR + 11
If you give a Boss legendary actions when it didn't have those before, its CR is increased by 3.
Legendary Actions that cost 1
Attack. The Boss makes one attack, even if it has the ability to multiattack.
Inflict Condition. Each creature within 5 feet of the Boss must succeed on a saving throw or be either blinded (Constitution), deafened (Constitution), frightened (Wisdom), paralyzed (Wisdom), poisoned (Constitution), or restrained (Strength) until the end of the creature's next turn.
Cast Cantrip. The Boss casts a cantrip from its list of known cantrips.
Coordinated Assault. (Requires a minion) The Boss makes an attack, and then one of its minions the Boss can see within 30 feet of it can use its reaction to make a melee attack.
Detect. The Boss makes an ability check.
Move. The Boss moves 30 feet without provoking opportunity attacks.
Legendary Actions that cost 2
Ascend. (Requires a flying speed) Each creature within 10 feet of the Boss must succeed on a Dexterity saving throw or take damage and be knocked prone. The Boss can then fly up to half its flying speed.
Aura of Negative Energy. The Boss creates an aura in a 60-foot radius, which spreads around corners. Each creature within can’t regain hit points. A strong wind disperses the aura, which otherwise disappears at the end of the Boss' next turn.
Cast Spell. The Boss casts a spell up to 3rd level from its list of prepared spells, using a spell slot as normal.
Energy Drain. One creature affected by a condition from the Boss takes damage, and the Boss regains hit points equal to the damage the creature takes.
Harmful Shield. The Boss receives temporary hit points equal to four times its damage that last until the start of its next turn. Until all these hit points are gone, any creature that touches the Boss or hits it with a melee attack takes damage.
Shield. The Boss creates a magical field around itself or another creature it can see within 60 feet of it. The target gains a +2 bonus to AC until the end of the Boss' next turn.
Teleport. The Boss magically teleports, along with any equipment it is wearing or carrying, up to 120 feet to an unoccupied space it can see.
Improved Attack. (Requires multiattack) The Boss uses its multiattack feature.
Incorporeal Dash. The Boss magically transforms into an incorporeal form, moves up to 60 feet, and reverts to normal. While in this form, the Boss is immune to all damage, and it can't be grappled, petrified, knocked prone, restrained, or stunned. Equipment worn or carried by the Boss remain in its possession.
Inflict Damage. Each creature of the Boss' choice within 10 feet of it must make a saving throw. Each creature takes damage on a failed save, or half as much on a successful one.
Inflict Lasting Condition. A creature of the Boss' choice within 10 feet must succeed on a saving throw or become blinded (Constitution), deafened (Constitution), frightened (Wisdom), paralyzed (Wisdom), poisoned (Constitution), or restrained (Strength) for 1 minute. The affected target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a target's saving throw is successful or the effect ends for it, the target is immune to the chosen effect when inflicted by the Boss for the next 24 hours.
Inflict Mayor Condition. Each creature within 10 feet of the Boss must succeed on a saving throw or be charmed (Wisdom), incapacitated (Constitution), or stunned (Intelligence) until the end of the monster’s next turn.
Legendary Actions that cost 3
Aura of Harm. The Boss creates an aura in a 60-foot radius, which spreads around corners. The area is heavily obscured to creatures other than the Boss. Each creature other than the Boss that ends its turn there must succeed on a saving throw, taking damage on a failed save, or half as much on a successful one. A strong wind disperses the area, which otherwise disappears at the end of the Boss' next turn.
Cast Greater Spell. The Boss casts a spell up to 6th level from its list of prepared spells, using a spell slot as normal.
Heal Self. The Boss magically regains hit points equal to its damage.
Inflict Eternal Condition. A creature of the Boss' choice within 30 feet must succeed on saving throw or become blinded (Constitution), deafened (Constitution), paralyzed (Wisdom), or poisoned (Constitution) until magic such as the lesser restoration spell removes the condition.
Inflict Greater Damage. Each creature of the Boss' choice within 20 feet of it must make a saving throw. Each creature takes damage equal to two times its damage on a failed save, or half as much on a successful one.
Optional Rule - Mythic Traits
Recharges after a Short or Long Rest
If the Boss is reduced to 0 hit points, it doesn’t die or fall unconscious. Instead it gains one of the effects listed below. Additionally, the Boss learns two extra legendary action options that it can use. Award the party twice the amount of XP for defeating the Boss after it uses its Mythic Trait.
Mythical Awakening. (Requires Legendary Resistances) The Boss regains hit points equal to half its hit dice rounded up, it recharges its attacks, and it regains any expanded uses of Legendary Resistance.
Mythical Protection. The Boss reveals its four inner weak points. A weak point has the same AC as the Boss and a quarter of the Boss' hit points. It has the same immunities as the Boss, and is immune to all conditions. If it is forced to make a saving throw, treat its ability scores as 10 (+0). If the Boss finishes a rest, the weak points regenerate and are hidden again. The Boss dies only when all its weak points are destroyed.
Mythical Revival. The Boss regains all its hit points.
Mythical Summoning. The Boss regains hit points equal to half its hit dice rounded up. In addition, it summons three minions with a CR 10 lower than itself, in unoccupied spaces within 5 feet of it. The summoned are under the Boss' control and act immediately after the Boss in the initiative order.
Optional Rule - Telegraphing
Instead of just dealing damage between player's turns, the Boss spends one of its Legendary Actions to prepare for a Legendary Action Option with a cost of 2 or more. If you play with facing rules, the Boss can use this moment to also turn itself to face in the right direction.
Whether you play theater of the mind or on a grid, it now becomes clear to your players what the Boss is winding up to do. If on a grid, you can highlight the area that becomes affected. When in theater of the mind you need only name the characters that are targeted.
After the next player has made its turn, the Boss spends the remaining Legendary Actions needed to do its action. Everyone within the area that the Boss has targeted automatically loses their saving throw against the attack or the Boss automatically meets the AC of all targeted creatures.
Doing this gives your players the chance to avoid being hit by actively shoving their friends out of reach, running away themselves, or shoving minions in the damaging area.