Archmage (5e Epic Boon)
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You are about to reach the pinnacle of magic, and perhaps you will be able to cross the barrier between mortals and demigods. Take your spellbook and tear the fabric of magic itself. Show the world that you can be its dominator. "Scientia est potentia."
Your Intelligence score increases by 2. This bonus has an Ability score maximum of 30.
You also gain a benefit depending on your school of magic (detailed below).
School of Abjuration: You shall protect what others can not. Large amounts of mana have merged with your skin to the point of making it better armor than those of the greatest Paladin. You will not fear any being who uses the knowledge of the arcane against you. Starting now, when you create your Arcane Ward, its hit points will equal 2 * (your character level + Intelligence score) modifier. Additionally, creatures that cast counterspell or dispel magic of lower level than your spells have disadvantage in the spellcasting score check.
School of Conjuration: You shall conquer what others dare not. Creatures from other planes will be at your service, and will fight for your glory if necessary. Your magic, on the other hand, offers them the power to serve you better than any army. The creatures you invoke with conjuration spells can add your proficiency modifier to their attack and damage rolls, and their AC can be your spell save DC unless theirs is higher. You can make a conjurated creature be your companion permanently once per long rest if it's of a CR equal to half your level by casting a ritual of 10 minutes, but if you try to make another creature your permanent companion, the last one disappears or dies.
School of Divination: You shall see what others will not. The future is just another sight under your eyes. Good luck is a construct that only you can manipulate. You can select 2 options from your Third Eye instead of one, you get another die for Portent, and you can choose one of the dies of Portent to be a Natural 20 or a Natural 1 after rolling.
School of Enchantment: You shall rule as no Emperor has. No creature will challenge your voice, for your voice is the authority of a true King. Everyone will bow down before you. The range of Hypnotic Look increases up to 30 feet instead and the duration increases up to a number of rounds equal to your Intelligence modifier. You can now affect creatures that are immune to charm, fear, stun or any other status caused by an Enchantment spell. To do so, you must make an Intelligence check against the level or CR of the creature. On a successful roll, the creature is affected by the spell. You can only do this 2 times between long rests.
School of Evocation: You shall be victorious like no other champion was. Arcane is just another weapon that some carry with mediocrity, but for you it is something else: it is the door to unimaginable power. No one will resist you. The range of a critical increases by one for Evocation Spells, and a Natural 1 can be considered critical when using damaging spells that require saving throw (and 2 with the increase in critical range). Additionally, Powerful Cantrip now affects all other damaging cantrips: when you miss an attack roll, the enemy suffers half of the damage of the cantrip instead of not taking any damage.
School of Illusion: You shall turn reality what it is not. Illusion is just a different way to represent reality. And under your fingers, it is as fluid as air. You can now use Illusory Being twice per long rest, and the duration of your Illusion Spells doubles. You can also cast 2 illusory spells at the same time as an action once every long rest. Additionally, for Illusion spells your DC uses 2 times your Proficiency Bonus (8 + 2 x your Proficiency bonus + your Intelligence Modifier.) if the creature has less Intelligence than you. And your illusions can’t be seen by True Sight.
School of Necromancy: You shall rule over life and death as no god ever did. Undeads prostrate themselves at your feet in every step you take. You are their master and lord, and they are your knights in black armor. You will reign over the lives of many and the death of all. You will be more than King and Emperor. Your Undeads are permanently under your control if their Intelligence Score is lower than yours. Additionally, the Undeads under your command receive twice your level in hit points, and your proficiency bonus can be added to their attack roll and their AC. And when you receive damage, you can instead transfer that damage to Undead creatures under your control that are near 30 feet of you. Besides that, they all know one cantrip of your choice, but must be of the Necromancy School, and they will use your Spellcasting Score.
School of Transmutation: You shall create what it was not born. Creation is not something that only Gods can do. Transform coal into gold, or death into life, because weaving the very reality itself is a feat that only you can do. When you use your Shapeshifter feature, you can use it to cast True Polymorph instead of Polymorph. Also, your Transmutation Stone can have 3 options instead of one, and you can use twice the Transmutation Master feature instead of only once.
War Magic (Xanathar's Guide to Everything): You shall dominate the battle as no warrior will. Warrior in blood and wizard in mind. You are a mixed blood that is anxious about the next battle, and your blood boils with the great power that runs through your veins. Deflecting Shroud now makes damage equal to your level, instead of half, plus your intelligence modifier up to as many creatures as your proficiency modifier. In addition, Arcane Deflection now gives you a AC of 2 plus half your Intelligence modifier rounded down or to a saving throw a 4 plus your inteligence modifier.