So, considering this is the latest added mechanic to the game, and I feel I missed the chance to properly explain it, I'm making this post to make some clarifications and give an insight into the current characters that can use this kind of skill.

How to get access to it
After you complete the end of Version 0.4.X, go talk to AO-chan and she will give you an Accessorie called Dark Crystal Pendant. This is the item that allows you to access the two types of Transformations: Darkside and Time Paradox.  But in order to access the Time Paradox one, you will also need the Watson Clockworks. 
The following is an extensive explanation of the skills and effects of all current Hololive Members that have access to this mechanic. You can also take a look at the images on top to have a visual of their basic stats.

• GAWR:  Requires Dark Crystal Pendant
  
  • Passive Skill: Bloodlust
    Gawr damage increases the lower her Hp goes
  • 30% Evasion Rate
  • 20% Critical Rate
  • +10% TP Regen
  • -10% HP Regen
  • -50% Thunder Resistance 
  • 100% Water Resistance
  • 50% Paralysis Resistance
  • 100% Sleep Resistance
  • -100% Confusion Resistance
  • Seal and Charm Immunity
  • Unique Skills:
    • Vicious Shark: +50 TP / 100% Evasion Rate and +100% Spd for three turns
• FLARE: Requires Dark Crystal Pendant and Watson Clockworks
  
  • 200% Target Rate
  • 50% Critical Rate
  • 2 Attacks for turn
  • Blind Immunity
  • 100% Charm Resistance
  • 20% Poison, Sleep, Paralysis, Seal and Confusion Resistance
  • Unique Skills:
    
    • Smile & Go!!:  Lowers the enemies' Fire Resistance, then gives Fire Absorption to all allies
    • Celestial Flaming Arrow: Deals Light and Fire Hybrid Damage to one enemy. This Skill cannot be avoided
    • Shiranui: Deals Light and Fire Hybrid Fire Damage to all enemies twice
    • Sea of Flames: Fire Magic Damage to all allies and enemies
• K. COCO: Requires Dark Crystal Pendant and Watson Clockworks
  Actually, this one is a little more complex: First, if you have the Pendant, Coco will become Kiryu-chan. If Kiryu-chan manages to survive for three turns, you will gain access to the REAL Transformation if you also have the clock
  • 30% Critical Rate
  • 15% Counter Attack
  • +10% TP Regen
  • 100% Crit Evasion
  • Immunity to Poison, Blindness, Paralysis, Charm, Seal, Confusion, Sleep, Bleed, Death Sentence, Death Curse, Stats and Elemental Debuffs
  • Unique Skills:
    • Next Meme!: Strikes one enemy twice, ignoring their Def and highly decreasing Spd for five turns
    • Rising Strike, Ryuu no Gotoku!: Physical Damage to all enemies, ignoring their Def. This Skill cannot be avoided
• OLIVIA: Requires Dark Crystal Pendant and Watson Clockworks
  • Passive Skill: Crimson Rose
    Olivia's damage increases each time she attacks, gaining up to 10 stacks. Then, once she gains 10 stacks, she can use her skill Deadly Strikes
  • 7% Evasion Rate
  • 5% Critical Rate
  • Unique Skills:
    • Graceful Fencing: Attacks twice a single enemy, increasing her speed by 25% for three turns
    • Deadly Strikes: Strikes one enemy seven times, ignoring their Def and lowering its Sp. Def and Spd for three turns
• G. KORONE: Requires Dark Crystal Pendant and Watson Clockworks
  • Passive Skill: Tanabata Goddess
    Korone absorbs Ligh Damage. All allies Mp cost is halved and Light Element deals +25% more damage
  • Unique Skills:
    • Tanabata Health Wish: Heals all allies up to Hp 50% of their Max HP and removes Blindness, Paralysis, Charm, Seal, Confusion, Sleep, Berserker and Death Sentence
• NNN RISU: Requires Dark Crystal Pendant 
  
  • Passive Skill: Dual Cast
    All of Risu's Magic Skills are cast twice per turn
  • -50% MP Consumption
  • -50% Wind Resistance
  • +20% Earth Resistance
  • +20% Paralysis Resistance

• ROSEN BLACK JACK: Requires Dark Crystal Pendant 
  • Passive Skill: Halloween Phantom
    Has extra +30% Evasion and grant other allies the Status Night of Halloween (Dark Elemental Damage is active and deals
    +25% more damage with Dark Element Skills)
  • +20% MP Regen
  • Unique Skills:
    Barrier of Shadows:  Gives +100% Evasion to all other allies for three turns