Within your custom disaster you can create custom entities and then using the summon action you can spawn them in during your disaster!
To create custom mobs you have to create another skeleton body for entities outside of the other skeleton bodies. Custom entities are completely optional and do not need to be included in your custom disaster so you can ignore this section if you don't want custom entities, this is what your YAML file should look like:
# * Your settings skeleton body with info about the disastersettings:# Settings info# * Your core body for defining events and actions inside levelscore:# Core info# Your custom items skeleton body if you decided to create one for your disasteritems:# Custom items# This is your custom entities section notice how it is outside of the settings and core skeleton bodyentities:customZombie:# * The type of vanilla mob that the entity is (refer to resources for a list of entity type names)type:zombie# Mobs custom name for death messages and name tagname:'&cCustom Zombie'# You can define equipment for your custom entityequipment:# You can reference your own custom items by name here or minecraft items (refer to resources for a list of item names)# 'coolSword' is a custom item that we defined in the custom items sectionmainHand:coolSword# 'strongerShield' is a custom item that we defined in the custom items sectionoffHand:strongerShieldhelmet:iron_helmetchest:iron_chestplatelegs:iron_leggingsboots:iron_boots# You can define drop chances for each equipment slot on your custom mobdropChances:# Drop chances for gear, 1.0 meaning will always drop and 0.0 meaning will never drop (default: 0.1)mainHand:1.0offHand:0.5helmet:0.1chest:0.1legs:0.1boots:0.1# Health of the mob (default is mobs regular health)health:30.0# Movement/fly speedspeed:0.3# Range it will follow targets in blocksrange:50.0# Knockback it dealsknockback:1.0# Base damagedamage:6.0# Resistance to knockbackresistance:3.0scaryMob:type:skeletonname:'&cCustom'equipment:mainHand:bowhelmet:chainmail_helmetchest:chainmail_chestplatelegs:chainmail_leggingsboots:chainmail_bootsdropChances:mainHand:1.0helmet:0.1chest:0.1legs:0.1boots:0.1health:15.0speed:0.3range:50.0knockback:1.0damage:6.0resistance:3.0