How competetive is a deck like this? And how do games usually play out? Love the idea of this deck but my LGS is really competetive so I wanna make sure it can hold it's own.

COL_Cashmoney I would say the list as currently built is not competitive at all. The deck is great for slower metas because it's dealing like 2-6 damage a turn pretty consistently so it puts a "clock" on the game. If the games only last 4-5 turns then it's not really doing much of anything. I guess it depends on what you mean by "competitive". If you're talking true cEDH like Prosh + Food Chain or Blood Pod; this deck won't ever really hold a candle up to those. If you mean more in lines of highly tuned 75% decks, I could see a chance of this deck being pretty decent.

Not sure how competitive this type of deck can be, but here are some suggestions if you wanted to make it more highly tuned:

Mana Crypt, Mana Vault, Grim Monolith, and Basalt Monolith - Fast mana is great at speeding up any deck. Basalt Monolith is definitely the weakest of the four, but I included it so that you can pair it with...

Rings of Brighthearth - Paired with Grim Monolith and Basalt Monolith lets you generate infinite colorless mana which you can use for Xantcha's activated ability. You can only take out one person this way before she comes back under your control and you have to sacrifice her, so cards like Ashnod's Altar, Phyrexian Altar, Phyrexian Tower, and Viscera Seer would be helpful inclusions if you want to go this route.

More draw & tutors to make the deck more consistent like Vampiric Tutor & Imperial Seal. I've never played with Ad Nauseam so I'm not sure how much I can recommend it; but after taking out all of the high CMC cards out of the deck I imagine it would be pretty great.

Also since this deck is pretty much an "aggro" deck, you can throw in cards like Phyresis or Tainted Strike to reduce the amount of damage you need to deal to take someone out. You could always try for one of those cards or a Grafted Exoskeleton onto Heartless Hidetsugu with a Glacial Chasm in play, but that's probably too slow.

Finally, cards that focus on resource denial like mass land destruction and discard can slow down the game so you have enough time to close it out like Blood Moon, Impending Disaster, Ruination, Stranglehold, Planar Void, Mindslicer, etc.

I think the best way to make this deck competitive is to throw in a bunch of stax elements that will disrupt your opponents' combos and slow the pace of the game down. This deck's current strategy wants to get some damaging permanents on the board and have them stick around for as long as possible. Kind of similar to a Brago lock, it doesn't matter how much damage you're doing a turn if your opponents can't respond to it. The difficult part of that though is finding enough stax cards to put in the deck (and drawing them consistently) and the sad fact that Xantcha's ability literally works to refill their hands. While Xantcha might be an interesting concept, you might have more luck making a more tuned version of Mogis, God of Slaughter since he helps with the resource denial plan too. I guess it just depends on whether you want an infinite mana sink or a card that actively helps keep your opponents' boards clear.

Let me know if this helps!

Whoops, guess I lost my time window to edit my last comment. The last things I wanted to mention were Pyroblast and Red Elemental Blast. Great cards for countering your opponents' counterspells.

If you were to choose a Rakdos commander, Vial Smasher the Fierce would be the most competitive simply because you have the option to pair it up with other partner cards to get access to more colors. As it stands, Xantcha isn't a bad commander; it's just hard to competitively build her when she refills your opponents' hands and is a mana sink for the entire table.