Source: Dungeon Masters Guide
An Epic Boon is a special power available only to 20th level characters. Characters at that level gain such boons only if the DM wants them to and only when they find it appropriate. Epic Boons are best awarded after the characters complete a major quest, or accomplish something else particularly notable. A character might gain an epic boon after destroying an evil artifact, defeating an ancient dragon, or halting an incursion from the Outer Planes.
The text of a boon addresses its user. Unless a boon says otherwise, a character can't gain it more than once.
When you miss with a melee weapon attack, you can choose to hit instead. Once you use this boon, you can't use it again until you finish a short rest.
As an action, you can cast the Misty Step spell, without using a spell slot or any components. Once you do so, you can't use this boon again until you finish a short rest.
When another creature that you can see within 60 feet of you makes an ability check, an attack roll, or a saving throw, you can roll a d10 and apply the result as a bonus or penalty to the roll. Once you use this boon, you can't use it again until you finish a short rest.
Your hit point maximum increases by 40.
You gain one 9th-level spell slot, provided that you already have one.
You stop aging. You are immune to any effect that would age you, and you can't die from old age.
When you take damage from any source, you can reduce that damage to 0. Once you use this boon, you can't use it again until you finish a short rest.
You can bypass the damage resistances of any creature.
You can add a d10 roll to any ability check, attack roll, or saving throw you make. Once you use this boon, you can't use it again until you finish a short rest.
You have advantage on saving throws against spells and other magical effects.
You can give yourself a +20 bonus to a ranged attack roll you make. Once you use this boon, you can't use it again until you finish a short rest.
You are immune to all diseases and poisons, and you have advantage on Constitution saving throws.
When you gain this boon, choose a plane of existence other than the Material Plane. You can now use an action to cast the Plane Shift spell (no spell slot or components required), targeting yourself only, and travel to the chosen plane, or from that plane back to the Material Plane. Once you use this boon, you can't use it again until you finish a short rest.
Choose one of your spells of 1st through 3rd level that has a casting time of 1 action. That spell's casting time is now 1 bonus action for you.
You can use a bonus action to regain a number of hit points equal to half your hit point maximum. Once you use this boon, you can't use it again until you finish a long rest.
You have resistance to bludgeoning, piercing, and slashing damage from nonmagical weapons.
You gain proficiency in all skills.
Your walking speed increases by 30 feet. In addition, you can use a bonus action to take the Dash or Disengage action. Once you do so, you can't do so again until you finish a short rest.
Choose one 1st-level sorcerer , warlock , or wizard spell that you can cast. You can now cast that spell at its lowest level without expending a spell slot.
You can cast any spell you know or have prepared without expending a spell slot. Once you do so, you can't use this boon again until you finish a long rest.
You have immunity to fire damage. You can also cast Burning Hands (save DC 15) at will, without using a spell slot or any components.
While completely in an area of dim light or darkness, you can become invisible as an action. You remain invisible until you take an action or a reaction.
You have immunity to lightning and thunder damage. You can also cast Thunderwave (save DC 15) at will, without using a spell slot or any components.
You have advantage on ability checks made to resist being grappled. In addition, you can use an action to automatically escape a grapple or free yourself of restraints of any kind.
You have truesight out to a range of 60 feet.
You gain a +10 bonus to Dexterity (Stealth) checks, and you can't be detected or targeted by divination magic, including scrying sensors.