Update!
If anyone's interested, I've updated every Ability ive ever posted in this thread so that they are now compatible with Essentials v18.1. All of my custom abilities are now found in the main post, with all installation instructions found there. Abilities I made that ended up becoming real abilities in later gens, or very similar (Armor Piercer, High Pitched, etc), were removed, since you can just use their official equivalents.
Abilities I've made to work exclusively for Pokemon Birthsigns have not been included, since the Birthsign script has not yet been updated for Essentials v18.1.
In addition to updating old abilities, I've also included a slew of new ones I've never posted here before, bringing the total of custom abilities to 50.
Here are a glimpse of some of the new content:
Ability: Sharpshooter
Effect: The user's non-contact moves cannot miss, and cannot be redirected.
Ability: Cold Blooded
Effect: The user resists Fire moves, and its Speed increases by 1 stage when hit by a Fire move. Speed is also increased in sunshine. The opposite holds true for Ice moves and hail.
Ability: Pack Leader
Effect: Boosts damage when in the lead position of the party.
Ability: Wild Tantrum
Effect: Gains the Rage effect when entering battle, raising Attack by 1 stage whenever the user is struck.
Ability: Night Terrors
Effect: Thrashes while asleep & strikes all nearby Pokémon.
Ability: Grave Robber
Effect: Immediately steals a fainted battler's held item.
Ability: Midnight Curse
Effect: Changes the user's form and ability based on the time of day. This is designed as an exclusive ability for Lycanroc.
Ability: Daylight Rush
Effect: Boosts damage if the user is the first to move. Midnight Curse changes into this ability when Lycanroc changes into its Midday Form.
Ability: Twilight Shift
Effect: The user's stats change at the start of each turn. Midnight Curse changes into this ability when Lycanroc changes into its Dusk Form.
Ability: Moonlit Frenzy
Effect: Immediately retaliates when struck with an attack. Midnight Curse changes into this ability when Lycanroc changes into its Midnight Form.
Ability: Astral Plane
Effect: All Pokemon have the Ghost-type added to their typing when any Pokemon on the field has this ability.
Ability: Petrifying Gaze
Effect: Adds the Rock-type to nearby foes upon entry.
Ability: Pheromone Lure
Effect: Opposite-gendered Pokémon may not escape.
Ability: Seed Parasite
Effect: Applies Leech Seed to an attacker that KO's the user with a contact move.
Ability: Spike Burst
Effect: When the user is struck by a contact move, a layer of spikes is added to the attacker's side of the field. Applies Toxic Spikes instead of the user is Poison-type.
Ability: Color Aura
Effect: The user gains one of several different stat boosts or buffs depending on the "color" of the directly opposing Pokemon. A Pokemon's color is set in its PBS data.
Ability: Bounty Hunter
Effect: If a rare Pokemon is present on the opponent's side, the user marks that Pokemon as a bounty target. If the user KO's the bounty target, the user gains bonus exp and money from this battle. The user will also claim the bounty target's held item.
Ability: Hive Mind
Effect: Whenever the user is targeted by an attack, it commands a nearby ally to take the attack instead. This is designed as an ability to be used by Vespiquen, to command its Combee drones to shield it from harm.
You can find more in-depth details about each of these abilities in the main post.