Madness. If a humanoid spends at least 1 minute within 60 feet of Yawgmoth, it must succeed on a DC 14 Wisdom saving throw or descend into madness. The madness is a character flaw that lasts until cured and gets worse the longer you are in madness:  

madness level
1: you get paranoid
2: you start hearing noises that aren't there
3: disadvantage on checks
4: movement speed decreases by 10
5: disadvantage on attacks
6: movement speed is halved
7: you hallucinate: DM rolls a d20: if 12 or less, you do not attack your teammate, if 12 or higher, you attack your teammate because you see them as yawgmoth
8: disadvantage of save throws
9: movement speed is reduced to 0
10: Lose 20 health at the end of your turn each time

Brawler. Yawgmoth is proficient with improvised weapons. In addition, when he hits a creature with a melee weapon attack on his turn, he can use his bonus action to attempt to grapple the target.

Brute. A melee weapon deals two extra dice of its damage when you hit with it (included in the attack).

Regeneration: you regain 30 hit points at the start of his turn. He can only be killed when the final blow is done by the virgin. Otherwise, he regenerates 150 HP

Detect Humanoids: you can magically sense the presence of humanoids up to 150 ft away. you know the direction and exact distance to the creature.

Fear Aura. players must attack or run if they see Yawgmoth, cannot use any other actions., creatures within 10 ft cannot heal

Turn Immunity. you are immune to effects that turn undead.

Legendary Resistance (2/Day). If Jason fails a saving throw, he can choose to succeed instead.

Multiattack: you make two melee weapon attacks as an action

Machete: Melee Attack: +10 to hit, reach 5 ft., one target. Hit: 15 (3d6 + 7) slashing damage.

Head Crush: Melee Weapon Attack: +10 to hit, reach 5 ft., one creature that you are grappling. Hit: 26 (6d6 + 5) bludgeoning damage. If this attack reduces a creature’s hit points to 0 or less, you crushe the creature’s head. The creature dies if it can’t survive without the lost head. 

Reaction Name. Enter the description for your action.

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

Attack. Jason makes an attack with his machete.

Hide. Jason takes the Hide action.

Teleport (Costs 2 Actions). Jason magically teleports, along with any equipment he is wearing or carrying, up to 60 feet to an unoccupied space he can see.

Enter the description for how mythic actions work for your monster here.

Mythic Action Name (Costs # Actions). Enter the description for your mythic action.