Clone is a hero skill possessed by Oceanus (Fean) and is a common skill for Fean TrialHeroes when in trials above level 45. The skill clones the enemy creature card of highest cost and then increases its attack and HP based on the skill level. If multiple cards are the same cost the game appears to clone the right-most card in play. As of a recent update, the clones can now copy the 4th skill of evolved creatures and the runes of the creature, as well as grant the creature with a level of Immunity equivalent to the level of Oceanus' skill. Due to this, it is highly recommended that you level this skill up.
Facing this skill can be very dangerous and difficult especially if your highest costing card has recycle or revive, as this can give enemy decks this ability which they might otherwise not have. Some people choose to hold back their recyclers from play so that they cannot be cloned in an attempt to mitigate this effect.