Graveyard Mayhem!!!

1. Emissary of the Grave (Fiend/Dark Lv.5 ATK/0 DEF/0)
This card gains 400 ATK for every monster in your Graveyard. This card cannot be selected as the target of any card effects unless there are no other monsters to target. This card loses 200 ATK during each of your End Phases. If this card's ATK reaches 0 due to this effect, destroy it. You can only control one "Emissary of the Grave" at any time.

2. Possessed Graveyard Exemplar (Fiend/Dark Lv.5 ATK/0 DEF/0)
You can Special Summon this card from your hand when an "Emissary of the Grave" you control is destroyed by a card effect (except its own). This card's original ATK is equal to the number of monsters that were in your Graveyard when it was Summoned this way x400. You can destroy one card on the field with this card's effect, but if you do you cannot conduct your Battle Phase this turn. This card cannot be selected as the target of any card effects unless there are no other monsters to target. You can only control one "Possessed Graveyard Exemplar" at any time.

3. Raging Graveyard Horror (Fiend/Dark Lv.5 ATK/0 DEF/0)
You can Special Summon this card from your hand when a "Possessed Graveyard Emissary" you control is destroyed. When you do, give it these effects : This card's ATK is equal to the number of monsters in both players' Graveyards x250. This card cannot be destroyed by the effect of a Spell or Trap card, and it can only be targeted by the effects of Effect monsters if it is the only face-up monster you control. You cannot Normal Summon, Flip Summon, or Special Summon any monsters while you control this card.

4. Fiend's Cunning (Spell)
Destroy one Fiend-type monster on the field and any other two cards on the field. This effect can be applied to an "Emissary of the Grave" or a "Possessed Graveyard Exemplar" if it is the only monster you control and your opponent controls at least 2 monsters.

5. Chaos Gate (Spell/Continuous)
A player must pay 500 Life Points, or roll a die and send a card to the Graveyard from their Deck in accordance to the result (1/6= monster, 3/4=Spell, 2/5=Trap) to execute an attack. If the latter effect is chosen and a player must send a type of card not in the Deck to their Graveyard, the attacking monster is destroyed without applying damage calculation.

6. The Emissary's Ambition (Tarp/Continuous)
A player may choose to discard a card from their hand to their Graveyard if they inflict any Battle Damage to their opponent.

7. Beckoning from the Deep (Trap/Continuous)
Any monsters that would be removed from play are instead sent to the Graveyard. During each of your Standby Phases, pay 300 Life Points (This is not optional. If your Life Points are less than 300, your Life Points will fall to 0.)

Basically, these cards focus on getting Monsters into the Graveyard and finding any way possible to activate the special effects of "Emissary of the Grave". Keep in mind that though this is a powerful strategy, the risks it comes with are just as powerful, particularly against decks that focus largely on Sychro Summoning (since a lot of it seems to revolve around the Graveyard). It's great to use against "Different Dimension" decks, and it makes a deadly combo with Necrovalley (though that also comes with BIG risks) and can create pure chaos when combined with "The End of Anubis". Just keep the monsters away from "Skill Drain"-- that will destroy this strategy faster than you can can say "toast".

Note: Why doesn't "Raging Graveyard Horror" have the effect of controlling one at a time? Mainly because it is next to impossible to control another that isn't rendered completely useless. Even is you Summon one face down and have it revealed, it will have 0 ATK and have no special effects whatsoever, as it can only use those effects when it's Special Summoned by its only other effect. Also note that "Possessed Graveyard Exemplar" only gains ATK when it is Special Summoned by its effect. Destroying more monsters and sending them to the Graveyard will not grant it extra ATK once it's already there.

Emissary of the Grave is very lackluster. It needs more going for it.
Exemplar is a bit better.
For GY Horor, "You cannot Normal Summon, Flip Summon, or Special Summon any monsters while you control this card." is an extremely debilitating effect, even with the defensive effects it has.
Fiends Cunning is okay.
Chaos Gate is extremely clunkily worded to understand fully.
Emissary's Ambition seems somewhat useful. Not sure what for.
Beckoning in the Deep would alter the effects of cards that banish themselves in the Graveyard or on the field as costs, making an extremely strong cycle that might pose a problem. Instead of pay you can say "lose 300 Life Points".

Yeah, I think some of the costs might be a bit hefty... "The Emissary's Ambition" serves as a support card to send monsters to the Graveyard and power up the monsters listed here.