Asgore

Legendary Resistance (3/Day). If Asgore fails a saving throw, he can choose to succeed instead.

Magic Resistance. Asgore has advantage on saving throws against spells and magical effects.

Special Equipment. Asgore carries a magic trident called the Mountain Trident and plate armor. In the hands of anyone other than Asgore,  the Mountain Trident is a +2 Trident.

King of the Underground. Asgore is immune to the charmed and frightened condition, and all Intimidation checks made against Asgore are made with disadvantage. When in combat, at Initiative count 30, all hostile creatures need to make a DC 17 Charisma saving throw. On a fail, the damage Asgore takes from their attacks is reduced by 8. A creature can redo the saving throw at the end of their turn. When a creature passes the check, they are immune to this ability for 24 hours.

Ruler of Monsterkind. All monsters that are not considered Humanoids have to make a Charisma check against Asgore’s spell save DC (DC 17). Creatures that have an equal or higher CR than Asgore are immune to this ability. This ability includes companions of players, such as drakes and familiars.  On a fail, the creature cannot willingly attack Asgore or harm him in any way. An affected creature can redo the Charisma check at the end of their turn. When a creature passes the check, they are immune to this ability for 24 hours.

Boss Monster. Asgore is a special type of monster regarded as a boss monster. When Asgore is reduced to 0 hit points, he is instead put at 1 hit point. Should Asgore be killed, he will turn to dust, only leaving his trident and armor in his place. All Monstrosities within 5 miles will immediately become hostile to Asgore's killer.

Innate Spellcasting. Asgore’s spellcasting ability is Wisdom (DC 17, +9 to hit with spell attacks). He can cast the following spells, requiring no material components.

At will: Aganazzar's Scorcher, Burning Hands, Dragon Breath, Flaming Sphere, Fireball, Fire Bolt, Produce Flame, Scorching Ray, Searing Smite

3/Day: Fire Shield, Wall of Fire

1/Day: Flame Strike, Fire Storm, Immolation

Multiattack. Asgore can make three trident attacks during his turn. He can replace one of them with either his Fire Barrage action or casting a cantrip.

Mountain Trident. Melee Weapon Attack: +13 to hit, reach 20 ft., one target. Hit: 15 (2d8 + 6) piercing damage and 3 (1d6) fire damage.

Fire Barrage. Asgore unleashes a large barrage of fireballs, targeting all creatures within 90 ft. of him. The targets have to make a DC 17 Dexterity saving throw. On a fail, the target takes 36 (8d8) fire damage. On a success, the target only receives half the damage.

Sweeping Trident (Recharge 5-6). Asgore’s eyes repeatedly glint blue and orange before striking all creatures within 90 ft. of him. Flip a coin. On heads, the attack is blue and all creatures that have used their movement speed since Asgore’s last turn take 30 force damage. On tails, the attack is orange and all creatures that have not used their movement speed since Asgore’s last turn take 30 force damage. Afterwards, Asgore can move within 10 ft. of a creature he targeted.

Asgore Dreemurr can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at one time and only at the end of another creature’s turn. Asgore regains spent legendary actions at the start of his turn.

Melee Attack (1 Action). Asgore makes one attack using his Mountain Trident.

No Mercy (2 Actions). Asgore casts Compelled Duel on one target he can see within range, requiring no concentration and lasting the full duration.

Flame Sorcery (1 Action). Asgore casts one of his at-will spells.