There is no card that can be directly cast or activated from the graveyard that grants mana - the closest option is Past in Flames followed by ritual spells, which means you need at least six mana to start with. However, if you are talking about decking yourself completely (say, with Oath of Druids and no creatures, or Hermit Druid and no basics), there are a few things you can do.
Any card that can impact the gamestate without having been drawn is dangerous. Wizards have never printed a card that generates mana directly that way.
The closest is the aforementioned Narcomoeba-Dread Return combination.
One I always, always keep my eye on is Songs of the Damned. The card's effect is inherently broken but the necessary support does not exist. If some form of flashback Reclaim is ever printed, Songs is the card I expect it to break.
There is a way to turn 5 mana into 20 mana in this way - Recoup targetting Songs of the Damned - but it's not good enough even though that should be an immediate win out of the graveyard (Songs lets you activate Soul of Innistrad from the yard, bringing back a 2 card creature combo and a Spellskite to protect it).
EDIT: I'm wrong, you need Past in Flames not Recoup; for some reason I thought Recoup could hit instants too.
Yes we have Yawgmoth's will, but I'd like to imagine a deck where we deck ourselves and can get mana from it.
1) Put 3 Narcomoebas into play, then sacrifice them to flash back Dread Return. Depending how much and what type of mana you're looking for, you can target something like Priest of Gix/Priest of Urabrask, Sharuum, the Hegemon returning Gilded Lotus, Treasonous Ogre, Burning-Tree Emissary, Rasputin Dreamwaver, Workhorse, etc. If you have 1W available, reanimating Auriok Salvagers lets you Lion's Eye Diamond infinitely (the "Bomberman" win)
2) Best with Oath, Memory's Journey on your upkeep after milling out, putting back something like Black Lotus, Yawgmoth's Will, Lion's Eye Diamond. Flashback Deep Analysis to draw the two you don't, then cast both mana rocks, crack Lotus for Yawg's, crack LED in response, recast both, crack them both, and you have access to 9 mana and the entire contents of your graveyard
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The closest is the aforementioned Narcomoeba-Dread Return combination.
One I always, always keep my eye on is Songs of the Damned. The card's effect is inherently broken but the necessary support does not exist. If some form of flashback Reclaim is ever printed, Songs is the card I expect it to break.
There is a way to turn 5 mana into 20 mana in this way - Recoup targetting Songs of the Damned - but it's not good enough even though that should be an immediate win out of the graveyard (Songs lets you activate Soul of Innistrad from the yard, bringing back a 2 card creature combo and a Spellskite to protect it).
EDIT: I'm wrong, you need Past in Flames not Recoup; for some reason I thought Recoup could hit instants too.