Oh, look. A reference nobody has ever done.
To clarify, this article will be about "Kyūkyoku Jikaishin Sefiron", or, has it has been tentatively named, "Ultimate Temporal Machine God Sephiron". While you can read it's effect and statistics here, let me save you the effort.
Level 10 LIGHT Fairy4000ATK/4000DEFThis card cannot be Normal Summoned or Set. This card cannot be Special Summoned, except by having 10 or more Monster Cards in your Graveyard. Once per turn, you can Special Summon 1 Level 8 or higher Fairy-Type monster from your hand or Graveyard. Its effect(s) is negated, and its ATK becomes 4000.
Now, I doubt I'd miss the mark too much if I said that most would look at that summoning condition and relegate Sephiron to the unused card pool, where it can shake hands with cards such as Trent, Shapesnatch and Red-Eyes B. Dragon.
However, let us not be hasty. Firstly, let us look at the positive points. Firstly, it does not require a Normal Summon and there is no cost associated with it at all. Even "free" cards such as Chaos Sorcerer have a summoning cost, albeit a minor one. Secondly, it has an equal attack and defence, that of four-thousand apiece - there's no easy way to overcome it through beating it, and it tramples everything in it's path anyway. Thirdly, it's a Fairy and thus can aid other Fairies, most prominently Archlord Kristya and Master Hyperion. Fourthly, it can summon a Level 8 Fairy, negate it's effect and turn it into a 4000ATK beatstick.
No cost. No penalty. Nothing. Entirely of it's own volition, it can hit the field and call forth a 4000ATK monstrosity as a pal. You don't even have to use a Normal Summon. Why, if you cleared the field beforehand, it's a victory.
But now, we must look to that ever-annoying condition. Ten monster cards in the graveyard. This isn't something that can use it's effect on the first turn, before the opponent has set-up. No, this is almost late-game. For most decks, that is.
I'm sure that we are all familiar with the Lightsworn archetype? It's very foundation is based around using it's own deck as a candlestick to burn. I am sure that you see the synergy; the Lightsworns fulfill the summoning condition, and Sephiron clears up. Judgement Dragon can even sweep the field for Sephiron's arrival.
But, ah-h-h-h, the question is how will Sephiron push for game? Even with Judgement Dragon by it's side (something which is not guaranteed), the total falls a thousand short. The answer is, of course, a rather common tech-in for Lightsworns. None other than Archlord Kristya, of course - Kristya's incorporation into the archetype is often given the name "Kristyasworn" (I'd go with "Archsworn", myself...), but is not terribly popular.
With Sephiron, you can achieve an alternate win condition. Truth be told, I doubt you'd run many of them - Monster Reincarnation and Beckoning Light can retrieve Sephiron from the graveyard (the latter can also secure Honest, which would allow you to deal four-thousand points of damage with Sephiron without needing monster removal beforehand) should it get milled, and simply drawing into it puts it in your hand as well. I would think running two Kristya's and Sephiron would be sufficient; it's small enough to fit into the average Side Deck, so you can always keep it as a just-in-case option.
But I feel I'm rambling. Other ideas that occurred to me was the idea of a DARK build focused on Sephiron and a Darklord; the obvious candidate is Darklord Zerato, since it's effect is occasionally of use. To fill the grave for Sephiron's effect, the thought occurs that TeleDAD-esque plays would be advisable. Dark Grepher, Armageddon Knight, Destiny Hero - Malicious - things of that sort. You could also use Dark Armed Dragon, but I feel that the removal would make it hard to achieve that ten-monster requirement - this is also why I think Sephiron might have trouble fitting into Agent Angels, who have Hyperion. While the deck does fill the graveyard easily (Mystical Shine Balls/Agent of Creation - Venus for one), the cards would often be better spent destroying things through Hyperion's effect.
To conclude (at last!), should Sephiron see play, it would likely be in a Lightsworn deck, with an outlying possibility of a Darklord-orientated deck.