Blackheart

Blackheart cards are an archetype of cards primarily used by Kan which focus around using the abilities of the Effect monsters and Spell cards to summon the powerful Xyz monsters. Their appearance seem very reminisant of demonic creatures, specifically the wendigo. They have a sub-archetype known as Whiteheart which is used by Kane's counterpart Abal.

Design
Blackheart cards have a variety of designs but the majority of them have a demonic design which is reminisant of stags and deers, many of the Xyz containing some sort of horn motiff. The Effect monsters in the series have very human designs, appearing to be designed after fallen angels. The Xyz monsters have more variety to their design, some being very human looking and others appearing as monsterous creatures.

The monsters also share having the names of Fallen angels.

Playing Style
Blackhearts focus around utilizing the abilities of the Effect Monsters to bring out the Xyz monsters as well as using the Spell Card Blackheart Cycle to bring out a new, more powerful Xyz monster every turn. The lower ranked Xyz monsters and many of the effect monsters don't have very high ATK but useful effects that help to bring out the more powerful Xyz monsters.

The Effect monsters all have a shared ability.

"This card cannot be destroyed in battle. You take no battle damage from battles involving this card. This card is destroyed during your Second End Phase after this card has been Summoned."

This makes the Effect monsters very versatile. You can choose to either keep them on the field to stall your opponent and build your defense for a turn then use their other abilities to bring out and Xyz or focus on attacking with them to deal damage before Xyz summoning a more powerful monster.

The Xyz monsters also have a shared ability.

"This card cannot be destroyed by battle while it has Xyz material. When this card has no Xyz material, destroy this card."

The higher Rank Xyz monsters have additional effects that also destroy other cards on the field. The effects of most of the Blackheart Xyz monsters are to destroy as many cards the opponent controls as possible while the higher ranked Xyz monsters go even further by banishing them instead. This coupled with the fact that none of the monsters can be destroyed while they have Xyz material makes them very powerful and difficult to get rid of once they're out on the field.