Tree (Beta 20.0)
The game of Undercards has many unique abilities that come in two forms: Abilities attached with monsters and Effect Cards.
Ice Cap (Beta 6.5)

Effect Cards

Spider 9
Effect cards, also known as Spell Cards, don't require a spot on the board to be used. There are 52 Effect Cards currently in-game, 7 for each of the 7 classes and 3 others.
Screenshot 105
The Effect cards for the classes are devided in 3 Common, 2 Rare and 2 Epic in rarity. The other 3 cards are Thundersnail (Rare), Gaster Blaster (Legendary) and Load (DT), which can only be obtained through Snail Trainer, Sans/Gaster and Omega Flowey respectively.
Sad Dragon (Beta 16.0)

Monster Abilities

Charles (Beta 14.0)
Almost all monsters have an ability, each different from another. There are quite a few different methods of ativating these abilities:
Clam Boy (Beta 21.0)
  • Can't attack: Monsters with this ability can not attack during their turn, even if they have 1 or more ATK. They are still able to do that damage while defending.
    • It's the only negative ability, and will be removed by Asriel.
      Candy Dish (Beta 16.0)
  • Charge: These monsters can attack right after being placed. Other monsters need to wait until the next turn in order to attack.
    Punk Hamster
  • Haste: This works the same as Charge, but monsters can't attack the enemy directly on the first turn.
    Crazy Bun (Beta 14.0)
  • Taunt: Enemy monsters must target these kind of monsters. They can't attack any other monster or the player. Effects and abilities are unaffected by it.
  • Dodge: These monsters will take no damage the amount of times shown on the card. The number will decrease by 1 whenever they would take damage. This number will reset at the start of the owner's turn.
    • In a maintenance during Beta 14.1, Dodge was changed to reset at the start of both players' turn, but it was undocumented and was considered a bug.
    • Beta 18.0 reversed it back, but it was undocumented again.
    • Beta 19.0 pulled the same thing as in Beta 14.1, returning it to resetting at the start of both players' turn with an undocumented change.
    • Beta 20.0 changed it back again, but is now noted in the patch notes.
      Gaster Follower 1
  • Armor: Monsters with this effect will have the damage they take reduced by the number shown on the card.
    Bomb (Beta 19.0)
  • Transparency: Monsters with this effect are unable to be targeted by spells, effects and monsters. This effect lasts until the monster with the ability attacks. Taunt doesn't work if a monster has both effects.
    Fishing Rod
  • Candy: Monsters with this effect restore 3 HP at the end of their turn.
    • There are currently no monsters with just this effect.
      Ferry (Beta 21.0)
  • Turbo: These effects activate the moment the card with the effect is drawn from your deck.
    • This effect existed before the keyword, in the form of Mines.
      Scarf Mouse (Beta 19.0)
  • Magic: These activate the moment you place them from your hand. Summoned monsters from cards like Igloo and Lamp don't activate it.
    • This used to be Battlecry, followed by Enter.
      Snowman (Beta 6.7)
  • Future: These monsters have an ability that will activate after the number of turns shown on the card, like a delayed Magic effect. Silencing them won't stop the effect from activating.
    Poison (Beta 9.3)
  • Start of turn: As the name implies, these abilities activate at the start of every turn as long as the monster is alive and isn't Silenced.
    • This used to be "At the start of your turn, ...".
  • End of turn: These abilities function simmilar to the previous one, but will obviously activate at the end of every turn. This includes the turn in which they have been placed.
    • This used to be "At the end of your turn, ...".
  • Dust: These activate once the monster that has it dies. Returning these monsters to the owner's hand or deck with cards like Doggo, Endogeny and Memory Head won't activate them. They also won't be activated if they die while being Silenced with the help of cards like Scarf Mouse or Nacarat Jester.
    • This used to be Deathrattle, followed by Death.
  • Others: There are also abilities that don't follow any of these rules, but have their own way of triggering. Abilities like these include 'Whenever this monster is damaged...' or 'Whenever a monster is summoned...'.

Status Effects

With all this in mind, some spells and monsters (and even the passive effect of some Souls) can inflict a Status Effect on other monsters, all of which are negative and thus removable by cards like Asriel, just like "Can't attack":

  • Silence: If a monster gets Silenced, it loses all its abilities and will be reset to its base ATK and Max HP. However, if its Current HP is lower, it will not be restored to full health.
    • Determination monsters are immune to this effect.
    • Silence will also remove the following 2 status effects.
  • Paralyze: When a monster is paralyzed, it is unable to attack on its next turn.
  • KR: Monsters with this effect will take 1 damage at the start of their turn. Dodge resets before they take damage, and Armor blocks it completely.
    • This status effect is currently exclusive to Perseverance.