Here is a sample Mythic Mob. Feel free to copy it and alter it. Send your altered version to me. After I load the script you will be able to create an invisible mob spawner that spawns your mob. Please read my notes underneath the sample script.

SkeletonKing:
  Type: WITHER_SKELETON
  Display: ‘&6Skeleton King’
  Health: 500
  Damage: 10
  Skills:
  – message{cooldown=20;m=”<mob.name><&co> None may challenge the Skeleton King!”}
  @PlayersInRadius{r=40} ~onCombat >0 0.2
  – message{m=”<mob.name><&co> Ahahahahah! Die, <trigger.name>!”} @PlayersInRadius{r=40}
 ~onPlayerKill >0 1
  – skill{s=SummonSkeletons} @Self 0.1
  – skill{s=SmashAttack} @Target >0 0.2
  Equipment:
  – KingsCrown:4
  – SkeletonKingSword:0
 Drops:
 – SkeletonKingDrops
 Options:
 AlwaysShowName: true
 MovementSpeed: 0.2 
 MaxCombatDistance: 25 
 PreventOtherDrops: true

Note that in mythic mobs the skills list includes two different types of things. There are short skills listed there fully. For example:

– message{cooldown=20;m=”<mob.name><&co> None may challenge the Skeleton King!”}
@PlayersInRadius{r=40} ~onCombat >0 0.2

The part I highlighted in orange describes what the skill does. In this case it shows a message. The blue tells what the target of the skill is. In this case it is players within a forty block radius. The pink shows when the skill is triggered. In this case it happens on combat, but only some of the time.

Any short skills need to have all three of these parts. They need to include what happens, to whom it happens, and when it happens.

Longer skills are stored separately. In this case the SkeletonKing does the longer skill named SummonSkeletons. This skill is described in a separate file. I’ll paste that here:

SummonSkeletons:
  Cooldown: 15
  Skills:
  – message{m=”<mob.name><&co> Arise, my minions!”} @PlayersInRadius{r=40}
  – delay 20
  – summon{mob=SkeletalMinion;amount=2;noise=5} @Self
  – delay 20
  – summon{mob=SkeletalMinion;amount=2;noise=5} @Self
  – delay 20
  – summon{mob=SkeletalMinion;amount=2;noise=5} @Self

Note that within the longer skill SummonSkeletons, it lists short skills. Each of these have a description of what will happen. Some of them have targets. They don’t need to have triggers because the trigger is when the SummonSkeleton is triggered.

I am using the color coding here to show you what the different parts are. You don’t need to use color coding in your script.

Here are a few more examples of longer skills that NPCs can have. You can write your own skill.

BurningWind:
  Skills:
  – effect:particlesphere{particle=flame;amount=200;radius=5} @self
  – ignite{ticks=500} @PlayersInRadius{r=5}

The full online manual for Mythic Mobs plugin is found here: https://www.mythicmobs.net/manual/doku.php

I encourage you to check out and explore the lists of options there. For example, you may get inspiration for a new mob by browsing the list of skill mechanics. As always, if you have trouble you can email me with a description of what you want to happen and I can help you.