"Abyss-" (アビス Abisu) is an archetype of WATER (and one LIGHT) monsters, some of them are also part of the "Mermail" archetype. Members have existed in the TCG and OCG since Spell of Mask and Labyrinth of Nightmare, but had no support until Abyss Rising.
"Abyss-" monsters like "Abyss Soldier" and "Abyss Warrior" activate their effects by discarding a WATER monster, while others like "Mermail Abyssgunde" and "Mermail Abysshilde" have their effects activated when sent to the Graveyard. This helps the player to have card advantage over the opponent by adding others monsters to the hand while manipulating the opponent's cards. It is also possible to combine with some of the "Atlantean" cards.