3 Part Deck:
We need three pieces to assemble our engine: 1) Our Commander, card; Syr Carah, the Bold needs to be on the battlefield. 2) We need to have a cost reducer or mana generator out (we can go for the win without one of these, but closing out a game requires a mana engine 9/10 times). 3) Access to a burn spells--either in exile with Carah, in our hand, or in our graveyard with Underworld Breach or Past in Flames.
1) Syr Carah, the Bold: Carah's ability reads as follows:
"Whenever Syr Carah, the Bold or an instant or sorcery spell you control deals damage to a player, exile the top card of your library. You may play that card this turn. {T}: Syr Carah deals 1 damage to any target" [Emphasis mine].
If we cast Lightning Bolt, then we exile 1 card. If we cast Flame Rift, we exile 4 cards since four players (ourselves included) were dealt damage by the spell. If we cast Fiery Confluence, we dig 9 cards deep since the spell dealt 2 damage to each opponent 3 times. Grapeshot is disgusting with a decent storm count. Remember how the lack of card advantage is one of the major barriers to burn in EDH since aggro struggles to generate cards proportional to the number opponents? Carah's ability gets STRONGER with more players. By casting efficient burn spells we can dig through our deck, looking for more gas to keep digging.
2) Mana Engines & Cost Reducers: So we have access to cards, but we need to able to cast them. Helm of Awakening and Ruby Medallion are stupid good in the deck since they make our cheap spells and rituals even cheaper! Birgi, God of Storytelling
is disgusting since she, in effect, renders all our spell cheaper and makes our 0 MV rocks into mana positive rituals. The two of our other strong mana engines are Runaway Steam-Kin and Neheb, the Eternal. Steam-Kin basically gives us a free Dark Ritual every three spells. If those spells only cost each, then they were basically free. Neheb serves a vital role in the 99. We can tap out in our first main phase casting piles of burn spells, only to have 10+ added to our pool in our second main phase. In addition to Neheb and Steam-Kin, we have all the best rituals from red. Not only do they help accelerate your game plan, but they can net mana as you cast them from exile, keeping your mana pool full and your storm count high. One last interaction worth noting, but Treasonous Ogre and a moderate storm count with Aetherflux Reservoir means that you can dump life to cast a spell and immediately gain it all back.
3) Access to Burn Spells: Seems like a no-brainer, but absolutely crucial for getting our deck into motion. No burn spell means no cards for Carah to exile. We have a free ping each turn to dig, as well as wheels and looting/rummaging effects. Cards like Flame Jab and Lava Dart can be cast from the graveyard--repeatedly, in the case of the former. We run roughly 20 burn spells ( and the majority of them dig at least two cards deep) so this shouldn't be too difficult to accomplish, but burn spells are necessary. Galvanic Relay and Grapeshot dig super deep with a moderate storm count.
Gameplay:
Early Game (T1-T3): Prioritize ramping into casting Carah. We want as many rocks as possible down as ASAP. Look to use rituals to get Carah quickly (Seething Song is amazing in your opening hand). Once Carah is down, start pinging to dig extra deep for additional card draw and ramp. Don't spend burn spells on early turns unless Carah is down or you need to nuke a board full of dorks and hatebears. Hoard those burn spells until you have a solid payoff.
Mid Game (T4-T6): Stabilize and look for a window to go off. By now, one of two boards states have occurred: 1) You have accelerated into the most developed board-state. 2) You are behind. (It's really hard be in the middle). In the former, expect to become the archenemy. Look for ways close out the game quickly (the control players will presumably have full hands with counters and removal). If Carah gets picked off or the win con gets countered/removed, step back and play it safe. In the latter situation, play it slow. Try to fly under the radar. Avoid big scary plays that would draw attention to you and allow the other players to duke it out while looking for an opportune moment to go off. Use Carah for minor card advantage, and sit on any big burn spells or Aetherflux Reservoir. Once you think you have an opening, spend resources like there is no tomorrow.
Late Game (T7+): Look to rebuild. If the game has dragged on this long, you are probably screwed. The control and battlecruiser decks have a well-developed boardstate that Carah can't really deal with. The deck is ill-suited for a game of attrition against Graveyard decks. If we flopped hard on our storm turn, we may have anywhere from 1/4 to 3/4 of our deck in exile. While it is tricky to pull off a win, it is doable. I don't have much advice to offer other than just creatively use what you have on hand.
Pivoting
If Carah gets hated out of the game, the deck can pivot around one of the three red legendries featured in the 99. Each legendary can function like a secret commander for the game, list is ordered from most preferable to least. NOTE: This is one more reason why Carah is awesome. We are not locked into any one burn strategy since our commander is just a draw engine. Thus, out gameplan can pivot into different approaches depending upon draws, matchups, the pod dynamics, and how expensive Carah costs to recast.
1) Birgi Pivot: Combo Storm. Birgi, God of Storytelling
paired with Grinning Ignus is strong enough that we can cobble together a storm finish. Infinite casts into a Aetherflux or Grapeshot for game, or Galvanic Relay into "drawing" your library to find Atherflux or Grapshot.
2) Torbran Pivot: Grind 'Em Out. Torbran, Thane of Red Fell makes every burn spell we run extremely scary. Little "1 damage to everything" spells becomes a "3 damage to all our of opponent's stuff." Sit back and loot/wheel to keep your hand stocked. With Torbran out, Fiery Confluence deals 12 to each opponent and each iteration of Grapeshot becomes a Bolt.
3) Neheb Pivot: Fireball Finish. Neheb, the Eternal can very good good at closing out games by turning a pile of mana into a damage when paired with an X spell like Rolling Earthquake. A big spell in the first main phase, followed with another in the second main phase can but opponents low enough to finish off with incidental Bolts and Neheb combat beats. If this is your angle, do everything you to conserve your life total since those X spells hit everyone--Neheb makes for a great blocker in this regard.