I don't like how it makes you keep track of something without any sort of physical reminder. Maybe it could be something more like Azor's Elocutors? Of course, that's been done already. Maybe you could lower the cost and make it an aura for players that causes them to lose?
An uncounterable, unbouncable, unkillable wincon? With that cost and the time restriction it could actually work. I mean, even if you combo this out turn 1, aggro can still beat you. Presumably control could stop the combo. In any event I doubt that combo would be happening in standard.

It counts each player end steps. If you combo this out turn one in modern, it beats all combo decks (around 85/15)and control decks(Barely exists in modern), and goes about 50/50 with aggro if you have any defense. In standard, it's easy to cast on turn three, then win turn 5 or 6. That's a the same speed as a all-in aggro deck, but beats control almost perfectly.

Without the alt cost, it's power level is fine, but it's boring. Why not make 10 5/5 indestructible, unblockable, hasty demons? More flavorful and fun.

Black Mythic Noncreature.

Sorcery

Separate all permanents target player controls into two piles. That player sacrifices all permanents in the pile of his or her choice.

Final Sunset
Sorcery
Exile x target creatures and/or lands. For each permanent exiled this way, its controller puts a Relic artifact token onto the battlefield.
As the sun set for the last time on Frostwynd, everything was frozen into indistinguishable blocks of ice.

Cyrani, the Moontender –
Planeswalker – Cyrani
{+1}: Put a 1/1 black Moonfolk enchantment creature token with lifelink and deathtouch onto the battlefield.
{−3}: Exile target nonenchantment permanent.
{−6}: Search your library for up to three cards and put them into your hand. Then shuffle your library.
{3}

Endless Night
Sorcery (M)
Target player skips his or her next turn.

Death World
Enchantment
At the beginning of each player's upkeep, that player sacrifices a creature.
: CARDNAME becomes a */* Lhurgoyf enchantment creature with power and toughness equal to the number of cards in all graveyards until end of turn.

Breath of Anjomeddon --
Sorcery (MR)

Destroy all creatures. Each player loses 5 life.

"Anjomeddon will protect his new world with deadly ferocity. He is the sun, he is the light, he is the burning heat of Starstill."
-Prayer from Illpyre Brother's Cult-

Wander the Frozen Wastes
Sorcery
Destroy all creatures. Target player discards a card for each creature destroyed this way.

Consumed by Darkness
Sorcery
~ cannot be countered.
You may pay , 5 life, and sacrifice 5 permanents instead of paying ~'s mana cost.
Target player loses after that player's next five end steps.

Setraxas, Mind Abolisher
Planeswalker - Setraxas
[+2]: Target player discards a card.
[-X]: Cast target instant or sorcery card in an opponent's graveyard with converted mana cost X without paying it's mana cost.
[-8]: Target player loses his or her draw step.
3

Black Mythic Noncreature.