So you might be looking at this and going, "wow, this looks a lot like an Approach deck with some black splashed in for no reason."  And you'd be right!  This is just an Approach deck with Chromium as the win condition instead of Approach because I wanted to play a big body, elder dragon win condition deck.

That being said, the deck plays pretty much the same as Approach:  play your counterspells and removals carefully to control the board and infuriate your opponent as you stop them from sticking to the board for 15 minutes, crushing their soul.  As unnecessary as the black may seem, Fatal Push and Cast Down do a really great job of replacing Seal Away for early game removal.  In addition, with so many kinds and colors of removal at your disposal, Torrential Gearhulk does a great job of giving you repeat value out of spells you play on curve (while playing a strong body on the board), so you don't have to worry about drawing as fast as you can to find another copy of a removal spell.

As for the main event, Chromium is an amazing creature and Nexus of Fate pairs really well with it.  Since Chromium has Flash, you can easily play this on a turn where your opponent expends all of their mana and then have Chromium ready to go on your turn.  If things don't work out this well, Chromium's mana-cost-free secondary ability allows you to dodge most removal spells by turning it into a Hexproof minion at the cost of being a 1/1. While you may find other cards in your hand, like Fumigate, will suffice as discards for this ability now that Chromium is out on the board, Nexus of Fate also works as a great substitute as it will get shuffled back into your deck instead of going to the graveyard.  If you don't have either of these, excess lands kept in hand work wonders as discards as well. Discard counterspells if you have to, since technically you're getting the same effect for free, but keep in mind that using a counterspell to keep Chromium a 7/7 when pushing for damage or blocking may be better than discarding a counterspell.  Nexus of Fate, while an expensive card, allows you to expedite your damage so you don't have to play around board clears as long if you think your opponent runs them.  Extra turns also prevent your opponent from untapping and having time to draw answers that would ignore Chromium being on board, like burn or a big buff card (although it's likely you'll still have a counterspell or two in hand to deal with some of those).

Possible Substitutions:

Disallow -> Cancel: I think Disallow is strictly better than Cancel for its ability to proc on ability triggers, but if you don't have Disallow, I think this would be a fine replacement in most cases.

Fumigate -> Yahenni's Expertise, Golden Demise:  I would only sub out one Fumigate for either of these as the life gain effect is very strong against aggressive, board-centric decks, but I think either of these would be good tradeoffs.  If you have the choice, I would go with Yahenni's first since it has a stronger effect and gives you more tempo.

Nexus of Fate -> Torrential Gearhulk, Approach of the Second Sun: I love Nexus.  It feels fun to use, it irritates your opponent, and it hard counters those mill decks that float around.  However, it can feel like a brick in your hand most of the time.  While TG isn't much cheaper, having Flash and being able to play a copy of any instant from your graveyard can give you huge tempo and a big body to swing around if you want to get on board earlier and push damage with a larger minion base. Or, you know, you could just pop these out for Approach and have a strong alternate wincon.

Cast Down, Torrential Gearhulk -> removal ad nauseam:  Cast Down seems like a very strong spell in this deck, as it removes a large amount of creatures in the game for very little mana, and it's an instant to top it all off.  However, I am running 4 copies of a card that needs black, with black being the least-represented color in my lands.  If running 4 copies of Cast Down seems inconsistent, consider replacing a couple copies for something like Seal Away or Take Vengeance, which fill a similar role but with more land support.  Similarly, if you don't have Torrential Gearhulk or find it being a brick, take the same considerations to lower your curve.

Lands -> other lands: I think the balance of lands I have here is pretty good, but I'm no land scientist.  If you think you need to change it up, do so.  Just try to stray away from putting in too many lands that strictly enter the battlefield tapped, as you'll often need the mana tempo to play your counterspells and removal.

This deck -> an Approach deck:  no, seriously.  Approach is a super strong archetype and this is a wannabe knockoff that loses color consistency to play a cool elder dragon.  I'm not saying my deck here is strictly worse, but it's hard to argue against a deck whose gameplan is to put the opponent on a turn timer the whole match before they just win.

Anyway, that's all I've got.  Enjoy!