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Deck BuildingHaving one of a card doesn't tend to be that great, as it doesn't give you good odds for drawing it. Any card/combo that you want to see during your game, such as a crypt doc + possession, you should have 3 of in a deck.
Cards that are late game cards are ones you can get away with have 2 of, but having 1 of them doesn't really help in deck synergy.
Cards that are extremely viable to the deck, such as Altar of Ix are likely to be targeted by your opponent. You need more than 1, even though they autodraw, unless you can defend it with extreme ease. You only need 1 basic building because of autodraw.
I'm assuming you only have 1 Fiend, Eye, Assassin, Claimed, because that's all you own, right? If you have more, add them.
Consider:
Buildings (8x)
2x Vapor Mahal
1x Graveyard
2x Foul Obelisk
2x Alter of Ix
1x Eye of Varexes
Creatures (20x)
2x Cinder Mogi
3x Crypt Doctor
1x Yalrinian Fiend
3x Flame Ent
2x Hell Cat
3x Red Imp
2x Lawn Gargoyle
1x Blind Assassin
3x Nether Plasma
Enchants (5x)
3x Possession
1x Planar Rift
1x Claimed with Blood
Spells (9x)
2x Gather Spirits
3x Lysis
2x Sunburst
2x Tremor