This deck is based of a core of a Dark World's deck, but has been adapted to work as a Fiend Deck in General.
This deck does not revolve around a Core Tactic, but knowing how to work your way around a Basic Dark World's Deck will help you greatly.
Now, Lets look at some of the cards that will make a good base for this deck.
Raviel, Lord of Phantasms: This card is always a good choice to have in a deck that revolved around Fiends, Summoning this card revloves around tributing fiends, and it summons a Token with every one of your opponents, Which helps greatly with our next card.
Goblin King: This is always going to be a good card in any fiend deck, This card gains 1000 ATK for every Fiend on the Field, This means, with a full field, You'll have 4000 ATK, This also applies to your opponents field aswell, meaning that you can reach up to 9000 ATK max.
Dark Ruler Ha Des/ Belial - Marquis of Darkness: These cards are different, but can be sorta grouped in the same tactic, Ha Des negates all Effects from Effect Monsters on your Fiend Cards, While Belial means that only can be target for anything, Spell, Traps, Effects. These will help keep your big monsters unharmed.
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Rec 1 : Darkness Neosphere - a nice, easy but powerful fiend
Rec 2 : Hate Buster - A nice and magic cylinder like trap